By Michael Donhauser

Des Moines, Iowa (dpa) - Candidates in the race for US president spent the weekend criss-crossing the state of Iowa ahead of Monday night's caucuses, where voters for the major parties will be making their first choices for their preferred nominee.

The 11 Republican and three Democratic candidates held rallies and shook hands across the rural Midwestern state.

For decades, Iowa has been first to vote in the state-by-state nominating process leading up to national party conventions in July. The general election is November 8.

Amid a lingering scandal over her handling of emails while secretary of state from 2009-13, former first lady Hillary Clinton remains the national front-runner for the left-leaning Democratic Party's nomination.

Yet she faces an early challenge in Iowa, where the latest survey published by the Des Moines Register newspaper gives her 45 per cent from likely Democratic voters in Monday's caucuses, in which meetings are held by party members in local precincts. Her main rival, US Senator Bernie Sanders of Vermont, was at 42 per cent.

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

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.