The vast majority of Americans say they are afraid of at least one of the two major candidates - the Democratic Party's Hillary Clinton and the Republicans' Donald Trump - winning the US presidential election, according to a new poll.

More than four-fifths (81 percent) of Americans say they would feel afraid following the election of one of the two presumptive US presidential nominees, a poll by The Associated Press-GfK said on Thursday. That figure includes a quarter who say it does not matter who wins: they are scared of both.

The online poll of 1,009 adults was conducted between July 7 and 11.

Three-quarters of voters say their pick for president is motivated by a desire to cast their vote against Clinton or Trump, more than those who say they are voting for the candidate who shares their positions on the issues or is the most qualified to hold the office.

"I really don't love either of the candidates. What do they say? It's a choice between hot and hell," said Annette Scott, 70, of Monmouth County, New Jersey.

Unfavourable views

Fifty-seven percent have an unfavourable view of Clinton, compared with 37 percent who have a favourable view.

Sixty-three percent have a negative view of Trump, compared to the 31 percent who think well of him.

READ MORE: Donald Trump's world views make world 'less safe' - Poll

Fifty-six percent of Americans said they would feel afraid and 48 percent say they would feel regret if Trump becomes president - in contrast to just 22 percent saying that a Trump win would make them proud and 26 percent excited.

Clinton does not fare much better: If she is elected president, 48 percent say they would be afraid and 46 percent say they would feel regretful. Only 27 percent of Americans would be proud of that choice, and 26 percent would be excited at her election.

The poll was conducted after the FBI recommended that Clinton should not face criminal charges for her use of a private email account and servers as secretary of state.

More than half of Americans said they believed Clinton broke the law by using private servers and a private email account for classified material.

READ MORE: US State Department reopens probe into Clinton emails

In recent weeks, Clinton has started to acknowledge that many voters simply do not trust her.

"I personally know that I have work to do on this front," she said in Chicago last month. "You can't just talk someone into trusting you. You've got to earn it."

Still, for Clinton, there was some good news in the poll: Nearly two-thirds of Americans think she will win the election.

Forty-seven percent of Trump's supporters consider him only slightly or not at all civil; 39 percent say he is slightly or not at all likeable; and 31 percent say he is only slightly or not at all qualified. Of those backing him, 14 percent consider him at least somewhat racist.

But even while they voice concerns about their own candidate, vast majorities of voters see the alternative as far worse.

Eighty percent of Trump supporters and three-quarters of Clinton backers say a major reason for their support is opposition to the other candidate.