The results are just the latest stunning numbers for Sanders as he seeks to topple Clinton, who is backed by virtually the entire Democratic establishment.
Sanders previously overtook Clinton in New Hampshire, another key early-voting state that neighbors his home state of Vermont.
A Sunday NBC/Marist College poll of New Hampshire also found Sanders with 41% support among Democratic voters, but with a wider, nine-point lead over Clinton, who had 32% of the vote.
Photos of Bernie Sanders on the campaign trail:
Bernie Sanders on the campaign trail
A striking new poll found Bernie Sanders surging past Hillary Clinton in Iowa
PHOENIX, AZ - MARCH 15: Democratic presidential candidate Sen. Bernie Sanders (D-VT) speaks to a crowd gathered at the Phoenix Convention Center during a campaign rally on March 15, 2016 in Phoenix, Arizona. Hillary Clinton won the Democratic primary elections in Florida, North Carolina and Ohio, while Missouri and Illinois remain tight races. (Photo by Ralph Freso/Getty Images)
DES MOINES, IA - JANUARY 26: Democratic presidential candidate Sen. Bernie Sanders (I-VT) speaks to the media after holding a campaign event with United Steelworkers Local 310L, on January 26, 2016 in Des Moines, Iowa. Sanders continues his quest to become the Democratic presidential nominee.. (Photo by Joe Raedle/Getty Images)
TOPSHOT - US Senator and Democratic Presidential Candidate Bernie Sanders speaks during a campaign event at the University of Northern Iowa in Cedar Falls, Iowa, January 24, 2016, ahead of the Iowa Caucus. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, participates in the Democratic presidential candidate debate in Charleston, South Carolina, U.S., on Sunday, Jan. 17, 2016. Hours before Sunday's Democratic debate, the two top Democratic contenders held a warm-up bout of sorts in multiple separate appearances on political talk shows, at a time when the polling gap between the pair has narrowed in early-voting states. Photographer: Patrick T. Fallon/Bloomberg via Getty Images
NEW YORK, NY - JANUARY 05: Democratic presidential candidate Sen. Bernie Sanders (I-VT) shakes hands with supporters after outlining his plan to reform the U.S. financial sector on January 5, 2016 in New York City. Sanders is demanding greater financial oversight and greater government action for banks and individuals that break financial laws. (Photo by Andrew Burton/Getty Images)
LEBANON, NH - NOVEMBER 11: Presidential candidate Bernie Sanders (I-VT) marches in the Veterans Day Parade November 11, 2015 in Lebanon, New Hampshire. Sanders goes into the Democrats second debate this weekend still running strong in the polls.(Photo by Darren McCollester/Getty Images)
Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, speaks at the Congressional Hispanic Caucus Institute conference in Washington, D.C., U.S., on Wednesday, Oct. 7, 2015. While next Tuesday's first Democratic presidential debate will probably lack the name-calling and sharp jabs of the Republican face-offs, there's still potential for strong disagreements between the party's leading contenders. Photographer: Andrew Harrer/Bloomberg via Getty Images
Democratic Presidential candidate Senator Bernie Sanders speaks about the Workplace Democracy Act on Capitol Hill in Washington, DC on October 6, 2015. AFP PHOTO/JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
US Senator from Vermont and Democratic presidential candidate Bernie Sanders addresses striking low-wage contract workers from the US Capitol and religious leaders at the Lutheran Church of the Reformation in Washington, DC, on September 22, 2015 for an interfaith service ahead of the arrival of Pope Francis for a six-day visit to the US. AFP PHOTO/NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
MANCHESTER, NH - SEPTEMBER 19: Democratic Presidential candidate Senator Bernie Sanders (I-VT) talks on stage during the New Hampshire Democratic Party State Convention on September 19, 2015 in Manchester, New Hampshire. Five Democratic presidential candidates are all expected to address the crowd inside the Verizon Wireless Arena. (Photo by Scott Eisen/Getty Images)
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Clinton jumped into the race as the overwhelming favorite for her party's nomination, but her campaign has endured a difficult summer.
Notably, she has struggled to address the controversy over her email use as secretary of state. Clinton exclusively used a private server for her State Department work between 2009 and 2013, raising questions over whether classified information was properly handled. The FBI is now reportedly looking into the issue.
Tuesday marked a sharp turnaround in how Clinton addressed the email flap. As recently as Monday, she was refusing to apologize and insisting her email system was appropriate. But she reversed herself and is now offering a direct apology to her supporters while admitting she made a mistake.
"I wanted you to hear this directly from me," Clinton said in her message to supporters. "Yes, I should have used two email addresses, one for personal matters and one for my work at the State Department. Not doing so was a mistake. I'm sorry about it, and I take full responsibility."
However, Democratic hand wringing over Clinton's campaign has continued to bubble up, especially because Sanders, a self-described democratic socialist, is not viewed as a strong general-election candidate.
Party leaders are increasingly looking at the potential for Vice President Joe Biden to enter the race. A New York Times story out Thursday even speculated about whether Secretary of State John Kerry, former Vice President Al Gore, or Sen. Elizabeth Warren (D-Massachusetts) could leap into the fray to stop Sanders.
Clinton's future isn't completely tied to New Hampshire and Iowa, however, and she has built up a gigantic fundraising edge and campaign infrastructure that would serve as a bulwark against any challenger, especially in later primary states that have more diverse electorates.
Notable moments from Hillary Clinton's 2016 campaign:
Hillary Clinton's 2016 campaign
A striking new poll found Bernie Sanders surging past Hillary Clinton in Iowa
BALTIMORE, MD - APRIL 10: Democratic presidential candidate Hillary Clinton holds a campaign rally at City Garage April 10, 2016 in Baltimore, Maryland. Voters will head to polling places for Maryland's presidential primary April 26. (Photo by Chip Somodevilla/Getty Images)
Democratic U.S. presidential candidate Hillary Clinton laughs as she listens to Representative Steve Israel (D-NY) speak on a gun control panel in Port Washington, New York April 11, 2016. REUTERS/Lucas Jackson
NEW YORK, NY - APRIL 09: Democratic presidential candidate Hillary Clinton holds a Latino organizing event on April 9, 2016 while campaigning in the Brooklyn Borough of New York City. The New York Democratic primary is scheduled for April 19th. (Photo by Andrew Renneisen/Getty Images)
SPRINGFIELD, MA - FEBRUARY 29: Democratic presidential candidate former Secretary of State Hillary Clinton speaks during a 'Get Out The Vote' rally at the Lyman & Merrie Wood Museum of Springfield History on February 29, 2016 in Springfield, Massachusetts. Hillary Clinton is campaigning in Massachusetts and Virginia ahead of Super Tuesday. (Photo by Justin Sullivan/Getty Images)
BOSTON, MA - Former Secretary of State Hillary Clinton speaks to supporters at the Old South Meeting Hall during a rally in Boston, Massachusetts on Monday February 29, 2016. (Photo by Melina Mara/The Washington Post via Getty Images)
MINNEAPOLIS, MN - MARCH 01: Democratic presidential candidate former Secretary of State Hillary Clinton greets patrons at Mapps Coffee on March 1, 2016 in Minneapolis, Minnesota. Hillary Clinton is campaigning in Minnesota as Super Tuesday voting takes place in 12 states. (Photo by Justin Sullivan/Getty Images)
MOUNT VERNON, IOWA - OCTOBER 7: Former Secretary of State Hillary Clinton speaks to voters during an outdoor town hall meeting at Cornell College in Mount Vernon, Iowa on Wednesday October 7, 2015. (Photo by Melina Mara/The Washington Post via Getty Images)
MUSCATINE, IOWA - OCTOBER 6: Former Secretary of State Hillary Clinton speaks to a voter before leaving a farm in Muscatine, Iowa on Tuesday October 6, 2015. (Photo by Melina Mara/The Washington Post via Getty Images)
MANCHESTER, NH - OCTOBER 05: Democratic presidential candidate Hillary Rodham Clinton holds a town hall meeting at the Manchester Community College on October 5, 2015 in Manchester, New Hampshire. Clinton spoke about the need for gun control on the wake of a mass shooting at another community college in Oregon. (Photo by Alfredo Sosa/The Christian Science Monitor via Getty Images)
DAVIE, FL - OCTOBER 02: Democratic presidential candidate Hillary Clinton speaks about gun control during her campaign stop at the Broward College Ã Hugh Adams Central Campus on October 2, 2015 in Davie, Florida. Hillary Clinton continues to campaign for the nomination of the Democratic Party as their presidential candidate. (Photo by Joe Raedle/Getty Images)
WASHINGTON, DC - SEPTEMBER 19: Hillary Clinton attends the Phoenix Awards Dinner at the 45th Annual Legislative Black Caucus Foundation Conference at Walter E. Washington Convention Center on September 19, 2015 in Washington, DC. (Photo by Earl Gibson III/Getty Images)
MANCHESTER, NH - SEPTEMBER 19: Democratic presidential candidate Hillary Clinton claps on stage during the New Hampshire Democratic Party Convention at the Verizon Wireless Center on September 19, 2015 in Manchester, New Hampshire. Challenger for the democratic vote Sen. Bernie Sanders (I-VT) has been gaining ground on Clinton in Iowa and New Hampshire. (Photo by Scott Eisen/Getty Images)
PORTLAND, ME - SEPTEMBER 18: Hillary Clinton brings her Democratic presidential campaign to Maine for the first time, speaking at King Middle School. Clinton is welcomed as she is introduced at the event. (Photo by Derek Davis/Portland Press Herald via Getty Images)
CEDAR RAPIDS, IA - SEPTEMBER 7: Democratic Presidential hopeful Hillary Clinton takes time to meet supporters and take photos at the Annual Hawkeye Labor Council AFL-CIO Labor Day picnic on September 7, 2015 at Hawkeye Downs in Cedar Rapids, Iowa. Clinton spent a busy Labor Day weekend in Iowa, meeting supporters throughout the state while trying to maintain a lead over Bernie Sanders for the Democratic nomination. (Photo by David Greedy/Getty Images)
US Democratic presidential candidate Hillary Clinton takes part in a discussion after speaking about the Iran nuclear deal at the Brookings Institution in Washington, DC, on September 9, 2015. Clinton expressed firm support for the nuclear accord with Iran, calling it flawed but still strong. Clinton added that the agreement must be strictly enforced and said that if elected president next year, she would not hesitate to use military force if Iran fails to live up to its word and tries to develop a bomb. AFP PHOTO/NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
PORTSMOUTH, NH - SEPTEMBER 5: Democratic presidential candidate Hillary Clinton and U.S. Senator Jeanne Shaheen take an off the schedule stop in the River Run Bookstore before shaking hands with onlookers on September 5, 2015 in Portsmouth, New Hampshire. (Photos by Charles Ommanney/The Washington Post via Getty Images)
PORTSMOUTH, NH - SEPTEMBER 5: Democratic Presidential candidate Hillary Clinton receives an endorsement from U.S. Senator Jeanne Shaheen (D-NH) September 5, 2015 in Portsmouth, New Hampshire. Clinton attended a Women for Hillary event at Portsmouth High School. (Photo by Darren McCollester/Getty Images)
PORTSMOUTH, NH - SEPTEMBER 5: Democratic Presidential candidate Hillary Clinton walks downtown Portsmouth and takes pictures with people September 5, 2015 in Portsmouth, New Hampshire. Clinton attended a Women for Hillary event at Portsmouth High School earlier in the day and received an endorsement from U.S. Senator Jeanne Shaheen (D-NH). (Photo by Darren McCollester/Getty Images)
LAS VEGAS, NEVADA - AUGUST 18: Former Secretary of State Hillary Clinton answers questions from journalists after speaking to north Las Vegas voters at a town hall meeting in Las Vegas, on Tuesday, August 18, 2015. The former Secretary was answering questions about emails sent and received a private server system, now in question, while she was the Secretary of State. (Photo by Melina Mara/The Washington Post via Getty Images)
UNITED STATES - August 15: Democratic presidential candidate Hillary Rodham Clinton greets fairgoers as she tours the Iowa State Fair in Des Moines, Iowa, on Saturday, August 15, 2015. (Photo By Al Drago/CQ Roll Call)
CARROLL, IA - JULY 26: Democratic presidential hopeful and former Secretary of State Hillary Clinton speaks to guests gathered for a house party on July 26, 2015 in Carroll, Iowa. Although Clinton leads all other Democratic contenders, a recent poll had her trailing several of the Republican candidates in Iowa. (Photo by Scott Olson/Getty Images)
CHICAGO, IL - MAY 20: Democratic presidential hopeful and former Secretary of State Hillary Clinton arrives for a meeting with parents and child care workers at the Center for New Horizons on May 20, 2015 in Chicago, Illinois. Clinton arrived in Chicago after campaigning Monday and Tuesday in Iowa. (Photo by Scott Olson/Getty Images)
LAS VEGAS, NV - MAY 05: Democratic presidential candidate and former U.S. Secretary of State Hillary Clinton (C) poses with students and faculty after speaking at Rancho High School on May 5, 2015 in Las Vegas, Nevada. Clinton said that any immigration reform would need to include a path to 'full and equal citizenship.' (Photo by Ethan Miller/Getty Images)
NEW YORK, NY - APRIL 23: Hillary Rodham Clinton (L) and actress Maggie Gyllenhaal attend the 2015 DVF Awards at United Nations on April 23, 2015 in New York City. (Photo by Michael Stewart/FilmMagic)
NEW YORK, NY - APRIL 29: Democratic presidential hopeful and former Secretary of State Hillary Clinton speaks during the David N. Dinkins Leadership and Public Policy Forum at Columbia University April 29, 2015 in New York City. Clinton addressed the unrest in Baltimore, called for police body cameras and a reform to sentencing. (Photo by Kevin Hagen/Getty Images)
Hillary Clinton announced her campaign for president on Sunday April, 12, 2015 with a video on YouTube.