WASHINGTON -- Most people who are drawn to third party candidates in the presidential election aren't sold on their choice, making these voters wild cards in an already unpredictable contest.

A shift in their support toward either of the major party nominees -- away from Libertarian Gary Johnson, Jill Stein of the Green Party or another third party candidate -- could drastically change the shape of the race.

A recent Associated Press-GfK poll found that nearly 7 in 10 third-party supporters say they could still change their minds.

They are about evenly split between Democrat Hillary Clinton and Republican Donald Trump if forced to choose between just those two. Nearly one-third refused to pick or said they would just not vote if it came down to that.

Margaret Bonnem, a stay-at-home mother in Colliersville, Tennessee, had previously supported Stein. But now she says she'll vote for Clinton because she realizes that "a third party candidate can't really do anything but pull votes away" from the major parties.

"I can't vote for Trump, and I don't want him to benefit from me voting for someone else," said Bonnem, 54. "So I'll end up voting for someone I don't fully trust."

The poll, conducted before last Monday night's first presidential debate, also shows signs that many third party backers would rather vote for no one than throw their support either to Trump or Clinton.

Among likely voters in the AP-GfK poll saying they'll support a third party candidate, 7 in 10 say they have an unfavourable opinion of both the Democratic and Republican nominees.

Altogether, the poll found 9 per cent of likely voters supported Johnson, 2 per cent Stein, and 2 per cent "another candidate."

Among third-party supporters, 72 per cent say Clinton's not at all honest, and 64 per cent say she's at least somewhat corrupt. Sixty-eight per cent say Trump is not at all compassionate and 59 per cent think he's at least somewhat racist.

Overall, 8 in 10 say they have an unfavourable opinion of each of the major party nominees. For 6 in 10, that opinion is very unfavourable.

Patrick Cannon, 63, from Minneapolis, says he'll vote for Johnson though he knows Johnson can't win.

"I guess my vote is in the nature of a protest vote," said Cannon, who retired from the graphics industry. "I just can't bring myself to vote for the other two."

These third party voters don't fit into easy political boxes.

They're disproportionately young: 26 per cent of them are under age 30, compared with just 15 per cent of likely voters overall. More than half of them self-identify as independents, though when asked which way they lean, they're about evenly split between the two parties.

They're deeply dissatisfied with the direction of the country. Eight-two per cent say the country is headed in the wrong direction, far closer to the percentage for Trump supporters (94 per cent) than Clinton supporters (45 per cent). Six in 10 disapprove of the job Barack Obama is doing as president.

Despite their dissatisfaction with the direction of the country, 74 per cent of them say they would be afraid if Trump is elected president, compared with 56 per cent who say that of Clinton. They're also slightly more likely to say they would be angry about electing Trump than putting Clinton in the White House, 54 per cent to 45 per cent.

Whether the Democrat or the Republican can win them over lends potential volatility to the race. In the AP-GfK poll, 69 per cent of them said they could still change their minds about whom to support, while more than 85 per cent of both Trump and Clinton supporters said their minds were made up.

Jim Staub, a retired captain in the Navy, is planning to vote for Johnson, but says his "leaning is weak."

"Not voting is a waste," said Staub, 75, of Laguna Niguel, California. "If I won't for Johnson, I will for one of them," meaning Clinton or Trump.

On average, surveys have suggested Clinton may perform slightly better when voters are forced to choose between only Trump and her.

But first, they'd need to actually vote. And there are reasons to think many of them wouldn't come out to hold their noses for their least-disliked candidate.

While 6 in 10 Trump and Clinton supporters in the AP-GfK poll say they always vote, just 45 per cent of third party voters say the same. In fact, more than a quarter straight up say they would not vote if they had to choose between Trump and Clinton.

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The AP-GfK Poll of 1,694 adults, including 160 likely voters who said they'll vote for a third party candidate, was conducted online Sept. 15-19, using a sample drawn from GfK's probability-based KnowledgePanel, which is designed to be representative of the U.S. population. The margin of sampling error for all respondents is plus or minus 2.5 percentage points, and for third party voters is plus or minus 8.2 percentage points.

Respondents were first selected randomly using telephone or mail survey methods and later interviewed online. People selected for KnowledgePanel who didn't have access to the internet were provided access for free.

Lemire reported from New York.