Candidates in the race for US president criss-crossed Iowa Saturday, speaking at rallies and meeting voters in a final push to win support ahead of all-important caucuses on Monday.

The 11 Republican and three Democratic candidates have less than two days to make their last appeals for votes before Iowa kicks off the state-by-state nominating process leading up to national party conventions in July and ultimately to the general election on November 8.

Former US secretary of state Hillary Clinton, once considered the clear front-runner among the candidates seeking the Democratic nomination, spent the day defending herself against charges that she mishandled emails while she served as the top US diplomat.

The scandal resurfaced this week after the US State Department said it would not make public 22 emails that it says are top secret.

The State Department said they were not marked classified at the time that they were sent, but her opponents said leaving them stored on her private email server was a security breach and said she should be prosecuted.

Clinton's campaign said their classification as top secret since she turned over her records "appears to be overclassification run amok."

Clinton was on the defensive while campaigning Saturday.

"This doesn't change anything about the fundamental facts," Clinton said, according to news reports. "I never sent an email marked classified."

Despite the scandal Clinton is the top pick of 45 per cent of likely Democratic voters in Monday's caucuses, according to a poll published Saturday by the Des Moines Register. Her main opponent, US Senator Bernie Sanders of Vermont, received 42 per cent.

Ray Lembsky, 37, a Clinton backer, told dpa he attended a Clinton rally on Saturday to show his support.

"Hillary has the best record in supporting workers," said Lembsky, a member of the Iowa Brick Layer Union. "We very much hope that she is going to be the next president of the US."

On the Republican side, front-runner Donald Trump had the support of 28 per cent in the poll, conducted with Bloomberg news. Second among the field of Republicans was US Senator Ted Cruz of Texas with 23 per cent; third was Senator Marco Rubio of Florida with 13 per cent.

Trump also spent time in Iowa on Saturday, landing in his private jet in Debuque to urge voters to attend the caucuses.

"Here's the thing," he told the crowd. "On February 1 you gotta get out and caucus. The bigger we can win by, the bigger the mandate. The more we can do."

Trump, a billionaire real estate mogul from New York, has played up his outsider status in vitriolic rants against the political establishment, his opponents and the media.

Republican National Committee member Steve Scheffler told dpa Trump has been successful because people are "sick of the good-old-boy-story."

He said Trump's candidacy wouldn't have been possible if the Republicans who have been elected would have behaved like Republicans. He also predicted that if there is a relatively high turnout of more than 130,000 voters, it would be in Trump's favour.

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