MONTPELIER, Vt. (AP) -- Vermont Sen. Bernie Sanders will announce his plans to seek the Democratic nomination for president on Thursday, presenting a liberal challenge to Hillary Rodham Clinton.
Sanders, an independent who describes himself as a "democratic socialist," will follow a formal statement with a major campaign kickoff in his home state in several weeks. Two people familiar with his announcement spoke to The Associated Press under condition of anonymity to describe internal planning.
Sanders will become the second major Democrat in the race, joining Clinton. He has urged the former secretary of state to speak out strongly about issues related to income inequality and climate change. The former first lady and New York senator is viewed as a heavy favorite in the Democratic primary and formally entered the race earlier this month.
Bernie Sanders through the years
Sources: Vermont Sen. Bernie Sanders to run for president
Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, speaks during an event in Iowa Falls, Iowa, U.S., on Monday, Jan. 25, 2016. With a week to go until the Iowa caucuses and the Democratic presidential race there in a virtual dead heat, Hillary Clinton and Sanders are mapping out divergent paths toward winning the first votes of the nomination process. Photographer: T.J. Kirkpatrick/Bloomberg via Getty Images
Washington, UNITED STATES: Newly-elected senators meet with Senate Minority Leader Harry Reid (R), D-NV, in Washington, DC 13 November 2006. From left are: Senator-elect James Webb, D-VA, Senator-elect Bernie Sanders, I-VT, Senator-elect Amy Klobuchar, D-MN, and Reid. AFP PHOTO/Karen BLEIER (Photo credit should read KAREN BLEIER/AFP/Getty Images)
US Congressman Elliot Engel (L) takes pictures next to US Senator Bernie Sanders after being dressed as Bouale leaders by public notaries of the Kouadioyaokro village, 150 km from Abidjan, 09 November 2008. US Senators Tom Harkin and Bernie Sanders visit comes ahead of a July 2008 certification deadline to ensure cocoa heading to the United States -- the third largest importer of Ivorian cocoa -- has not been produced with child labour. AFP PHOTO/ISSOUF SANOGO (Photo credit should read ISSOUF SANOGO/AFP/Getty Images)
COLUMBIA, SC - APRIL 25: Potential Democratic presidential candidate Sen. Bernie Sanders (R) (I-VT) delivers remarks at the South Carolina Democratic Party state convention April 25, 2015 in Columbia, South Carolina. Sanders joined former Maryland Gov. Martin O'Malley and former Sen. Lincoln Chafee in speaking to the convention. (Photo by Win McNamee/Getty Images)
WASHINGTON, DC - APRIL 20: U.S. Sen. Bernie Sanders (I-VT) participates in a 'Don't Trade Our Future' march organized by the group Campaign for America's Future April 20, 2015 in Washington, DC. The event was part of the Populism 2015 Conference which is conducting their conference with the theme 'Building a Movement for People and the Planet.'
(Photo by Win McNamee/Getty Images)
U.S. Senator Bernie Sanders, an Independent from Vermont and 2016 U.S. presidential candidate, greets supporters during a campaign rally in Madison, Wisconsin, U.S., on Wednesday, July 1, 2015. Sanders said he had attracted 200,000 donors as of mid-June and his campaign had raised $8.3 million online through June 17, according to FEC filings by ActBlue, the fundraising platform that he and some other left-leaning candidates and causes use. Photographer: Christopher Dilts/Bloomberg via Getty Images
PORTLAND, ME - JULY 6: Vermont Sen. Bernie Sanders speaks at the Cross Insurance Arena while campaigning in the Democratic presidential primary. (Photo by Derek Davis/Portland Press Herald via Getty Images)
Supporters hold up signs at a campaign rally for Senator Bernie Sanders, an Independent from Vermont and 2016 U.S. presidential candidate, in Madison, Wisconsin, U.S., on Wednesday, July 1, 2015. Sanders said he had attracted 200,000 donors as of mid-June and his campaign had raised $8.3 million online through June 17, according to FEC filings by ActBlue, the fundraising platform that he and some other left-leaning candidates and causes use. Photographer: Christopher Dilts/Bloomberg via Getty Images
PORTLAND, ME - JULY 6: Vermont Sen. Bernie Sanders speaks at the Cross Insurance Arena while campaigning in the Democratic presidential primary. Sen. Bernie Sanders greets supporters after speaking in Portland. (Photo by Derek Davis/Portland Press Herald via Getty Images)
PHOENIX, AZ - JULY 18: U.S. Sen. Bernie Sanders (I-VT) speaks to the crowd at the Phoenix Convention Center July 18, 2015 in Phoenix, Arizona. The Democratic presidential candidate spoke on his central issues of income inequality, job creation, controlling climate change, quality affordable education and getting big money out of politics, to more than 11,000 people attending. (Photo by Charlie Leight/Getty Images)
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Sanders plans were first reported Tuesday by Vermont Public Radio.
The white-haired senator and former mayor of Burlington, Vermont, has been a liberal firebrand, blasting the concentration of wealth in America and assailing a "billionaire class" that he says has taken over politics.
In recent weeks, Sanders has been a forceful critic of the proposed Trans-Pacific Partnership trade agreement, which would eliminate tariffs and other trade barriers for the U.S., Canada and Asian countries conducting commerce with each other.
"One of the key reasons why the middle class in America continues to decline and the gap between the very rich and everyone else is growing wider is because of disastrous trade agreements which have sent millions of decent-paying jobs to China and other low-wage countries," Sanders said.
He has called for universal health care, a massive infrastructure jobs and building program and a more progressive tax structure.