NEW YORK, July 8 (Reuters) - Americans' demand for an alternative to the two main presidential candidates has surged since the last election, a Reuters/Ipsos opinion poll shows, underscoring the unpopularity of Republican Donald Trump and Democrat Hillary Clinton.
Based on 2,153 interviews, Friday's poll results suggest a strong potential for a third-party candidate - like Gary Johnson of the Libertarian Party or Jill Stein of the Green Party - to take enough of the vote in the Nov. 8 presidential election to influence its outcome.
According to the July 1-8 poll, 21 percent of likely voters will not back Trump or Clinton. That compares with about 13 percent of likely voters who opted out of the two main choices at the same point in the 2012 race between incumbent President Barack Obama and Republican challenger Mitt Romney.
Click through images of Donald Trump's potential running mates:
Donald Trump's potential running mates, VPs
Donald Trump's potential running mates, VPs
Former Speaker of the House Newt Gingrich could provide Trump with exactly what he is looking for in a running mate — an experienced lawmaker who pushed legislation through Congress for years.
Though he has been actively aboard the Kasich bandwagon in recent days, Gingrich has come to Trump's defense regarding both the establishment backlash to his candidacy and the controversy the frontrunner found himself in after initially failing in a CNN interview to disavow support from former Ku Klux Klan leader David Duke.
(Joe Burbank/Orlando Sentinel/TNS via Getty Images)
Indiana Gov. Mike Pence
Pence is rumored to be one of the final few people on Donald Trump's short list to be running mate. He appeared with him mere days before Trump was expected to announce his decision, and even met with Trump's family.
Pence found himself in the spotlight in recent months after defending Indiana's religious liberty law that was criticized by many as being discriminatory against the LGBT community.
(Photo by REUTERS/John Sommers II)
A wildcard choice for sure, some began to wonder if Donald Trump might consider naming his daughter as his running mate after Sen. Bob Corker suggested the move shortly after taking himself out of the mix.
Ivanka, who would turn 35 mere days before the election, has not addressed the rumors, but brother Eric backed her.
(Photo by REUTERS/Carlo Allegri)
The 57-year-old retired lieutenant general has been advising the campaign on foreign affairs for months, but as Flynn's under-the-radar candidacy gained steam as Trump's decision drew near.
Conservative supporters have warned that Flynn isn't sufficiently tough on social issues.
(Photo by REUTERS/Gary Cameron/File photo TPX IMAGES OF THE DAY)
Gov. Chris Christie of New Jersey is the only 2016 GOP presidential candidate who has endorsed Trump since leaving the race.
Christie could help Trump with more moderate GOP voters, and he certainly has the bombastic personality that would serve as a useful surrogate for Trump, though the two also fiercely criticized each other when they were both candidates in the race.
Back in November, Trump said Christie could have a "place" on his ticket.
(Photo by John Moore/Getty Images)
Sen. Jeff Sessions of Alabama is the only sitting senator to endorse Trump — and he has already been tapped to lead Trump's national-security advisory committee.
"A movement is afoot that must not fade away," Sessions said during the Alabama rally where he announced his support last month.
Sessions is one of the staunchest supporters of Trump's hard-line plan to crack down on illegal immigration. The senator could also give Trump credibility in the South.
(Photo By Bill Clark/CQ Roll Call)
Former Sen. Scott Brown of Massachusetts was the first current or former senator to endorse Trump. He was known in the Senate as a moderate, and he could help pick up votes with some in the less conservative wing of the Republican Party.
He has supported abortion rights and is in favor of banning assault weapons, but he carries a blue-collar, populist persona. Brown memorably drove a pickup truck to campaign events during his 2010 Senate run in Massachusetts, which was to fill a vacant seat.
During a January event in New Hampshire, Trump said Brown was cut out of "central casting" and could be his vice president. Brown said at the time that Trump was "the next president of the United States."
(Photo by Keith Bedford/The Boston Globe via Getty Images)
"I was Donald Trump before Donald Trump became popular," Gov. Paul LePage of Maine said while announcing his support for the GOP frontrunner last month on "The Howie Carr Show."
The governor is comparable to Trump when it comes to provocative remarks. In January, LePage found himself at the center of a national firestorm after he made some racially tinged comments about out-of-state drug dealers who come into Maine and "impregnate a young white girl" before leaving.
"Now I get to defend all the good stuff he says," LePage has said of Trump.
LePage also entered politics after a successful business career, but he was reportedly staunchly opposed to Trump's candidacy before suddenly coming on board.
(Photo by Shawn Patrick Ouellette/Portland Press Herald via Getty Images)
Last week, BuzzFeed reported that advisers close to Huckabee thought the vice-president nod was in the cards for their guy.
Of all the former 2016 White House contenders, Huckabee may be closest to Trump ideologically. Huckabee struck a populist tone on cultural issues and, like Trump, vowed to protect Social Security and Medicare if elected.
(Photo by Christopher Furlong/Getty Images)
Aside from a few brushups in the fall, Gov. John Kasich of Ohio has barely touched Trump along the trail. The same can be said for Trump, whose most brutal attack against Kasich is that he "got lucky" because of the natural-gas reserves in his state.
It has been rumored that Trump would be interested in Kasich as his running mate, though Trump has also recently started criticizing Kasich on the campaign trail.
Kasich has the political experience that Trump says he's seeking. Kasich also hails from the Midwest, one of the most competitive regions in the past few presidential races.
(Photo by Scott Olson/Getty Images)
It has been an ongoing rumor that Gov. Rick Scott of Florida will endorse Trump after Scott wrote a gushing op-ed article in USA Today in January.
Like Trump, Scott rose to power from the business world. But Scott also has clout in the largest general-election swing state. In addition, he has six years of government experience behind him after being elected to office in 2010.
John McCain's running mate in 2008, Sarah Palin was a big get for Trump when she endorsed the frontrunner over Ted Cruz, whom she had vigorously campaigned for during his Senate run in 2012.
If Trump is interested in a sharp break with the Republican establishment, picking Palin would certainly send that signal.
It's an open question, however, as to whether she boosted or hindered McCain's run during the 2008 race.
(Photo by Patrick T. Fallon/Bloomberg via Getty Images)
Oklahoma Republican Governor Mary Fallin makes remarks before the opening of the National Governors Association Winter Meeting in Washington, in this February 22, 2014 file photo. REUTERS/Mike Theiler/Files
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The poll also showed a majority of American voters have an overall "unfavorable" view of both main candidates, with 46 percent of Clinton supporters and 47 percent of Trump supporters saying their top priority when voting will be to stop the opposing candidate from reaching the White House.
Demand for an alternative could be decisive in hotly contested battleground states. In Florida in 2012, for instance, Obama won by less than 1 percentage point. If this year's race is just as tight, third-party candidates could draw enough support to flip the state from one major party to the other.
Despite this, both Johnson and Stein have a problem that make their influence hard to predict - most voters still do not know who they are. Of likely voters, 23 percent say they are at least "somewhat familiar" with Johnson, a former governor of New Mexico. That drops to 16 percent for Stein, a physician.
Johnson could appeal to both liberals and conservatives. He wants to legalize marijuana and replace income and payroll taxes with a consumption tax.
Stein could make a strong bid to backers of U.S. Senator Bernie Sanders of Vermont, who ran a close race with Clinton for the Democratic presidential nomination. Stein wants to abolish student debt and raise the federal minimum wage to $15 an hour. She also aims for the country to run on renewable energy by 2030.
Stein's communications coordinator, David Doonan, said that the campaign is working to boost her numbers and that the Green Party is circulating a letter that directly appeals to people who supported Sanders. "He also started very low" in the polls, Doonan said.
So far it appears that Johnson and Stein draw support evenly from Clinton and Trump when they are included in opinion polls. In a four-way race, 45 percent of likely voters support Clinton, 34 percent Trump, 5 percent Johnson and 4 percent Stein, according to a separate five-day polling average on July 8.
That compares with 46 percent for Clinton and 33 percent for Trump in a two-way race.
Given a little more information about the two alternative candidates, respondents who back Johnson and Stein draw more deeply from Clinton's support.
Some 44 percent of likely voters support Clinton, 34 percent Trump, 7 percent Johnson and 5 percent Stein, after reading the following statement, according to the poll: "Jill Stein, the Green Party candidate for President, has taken an environmental position supporting a strong government role limiting carbon-based fuels, such as coal. Gary Johnson, the Libertarian candidate for President, has supported severely limiting the government's role, including slashing taxes and reducing programs such as Medicare and the military and broadly decriminalizing currently illegal drugs."
The Reuters/Ipsos poll is conducted online in English with American adults in the continental United States, Alaska and Hawaii. The survey of voters who want an alternative to Trump and Clinton included 2,153 likely voters and has a credibility interval, a measure of accuracy, of 2 percentage points. The five-day average poll that ended July 8 included about 1,240 likely voters and has a credibility interval of 3 percentage points.