Results in the first vote of the 2020 US presidential election have been embarrassingly delayed by what organisers described as “quality checks” and new reporting rules.

The Iowa Democratic Party said on Tuesday (Australian time) that results from the state’s first-in-the-nation caucus were greatly delayed, but the problem was not a result of a “hack or an intrusion”.

The statement came after Iowa voters packed caucus sites across the state, with at least four leading Democratic candidates battling to win the opening contest of the 2020 campaign, and ultimately, the opportunity to take on Donald Trump for the presidency.

Mr Trump’s campaign manager Brad Parscale said the delay in reporting the Iowa results was “the sloppiest train wreck in history”.

“And these are the people who want to run our entire health care system?” Mr Parscale said.

Despite the delays, candidates pressed ahead with post-election rallies.

“It looks like it’s going to be a long night, but we’re feeling good,” former vice-president Joe Biden told supporters, suggesting the final result would “be close”.

Senator Bernie Sanders said he had “a good feeling we’re going to be doing very, very well here in Iowa” once the results were made public.

“Today marks the beginning of the end for Donald Trump,” he predicted.

Voters earlier poured into more than 1600 schools, community centres and other public locations to render judgment on a field of 11 Democratic contenders led by front-runners Senator Bernie Sanders of Vermont and former Vice President Joe Biden, who have battled for the top in recent Iowa opinion polls.

Mostly white, rural Iowa is the first test in the state-by-state battle to pick a Democratic nominee to face Mr Trump in the November 3 election.

After more than a year of campaigning and more than $US800 million ($1.2 billion) in spending, the results in Iowa could begin to provide answers for a party desperately trying to figure out how to beat the businessman-turned-president.

The race has been overshadowed in recent weeks, with Mr Sanders, Elizabeth Warren and Amy Klobuchar relegated to part-time campaigning in Iowa as they were forced to remain in Washington for the Senate impeachment trial of Mr Trump. They heard closing arguments on Monday, just hours before the caucuses.

At the caucus sites, voters gathered in groups by candidate preference in a public display of support. If a candidate does not reach a threshold of support of 15 per cent of voters in a caucus needed to be considered viable, that candidates’ supporters are released to back another contender, leading to another round of persuasion.

Even if one candidate wins by a commanding margin in Iowa, Democrats may still lack clear answers as the race moves on to the other three early-voting states of New Hampshire, Nevada and South Carolina later in February.

Whoever remains in the race by Super Tuesday, when 15 states and territories vote on March 3, will also confront billionaire former New York Mayor Michael Bloomberg, who is skipping the early states in favour of focusing on states rich in delegates to the Democratic National Convention in July.

Mr Sanders, who finished in a virtual dead heat with Hillary Clinton in Iowa during his first presidential run in 2016, surged recently in many Iowa opinion polls to move just ahead of Mr Biden.

Ms Warren and Pete Buttigieg remain within striking distance, and many polls show a big bloc of undecided Iowa voters, creating the potential for upsets and late surges.

-with AAP