WASHINGTON, United States (AFP) – Republicans Donald Trump and Ted Cruz are neck and neck down the stretch to the first vote in the US presidential nominating contest, while Hillary Clinton struggles to repel a surging Bernie Sanders, polls showed.

With the Iowa caucuses 20 days away, the US presidential contest was on the boil yesterday, with several candidates barnstorming the heartland state and New Hampshire as they jockey for early bragging rights in the race to the November 2016 general election.

Celebrity billionaire Trump and conservative Senator Cruz are walking away with the Republican race in Iowa, but the pair are within a hair of one another, according to three polls released Monday and yesterday.

The latest survey by Quinnipiac University shows Trump leading with 31 per cent support to Cruz’s 29, while Senator Marco Rubio remains a distant third at 15 per cent and retired neurosurgeon Ben Carson is fourth with seven per cent.

Public Polling Policy’s survey yesterday shows Trump leading Cruz 28-26, with Rubio at 13 per cent, while ARG’s Monday poll has Trump with a four-point lead over Cruz, with Rubio in third.

“The Iowa Republican Caucuses are tight as a tick entering the final two weeks of the campaign,” Quinnipiac poll assistant director Peter Brown said.

Cruz fares better than Trump with voters on characteristics such as honesty, empathy, experience and shared values, he said.

“But they see the New York businessman as better able to handle some key issues. Trump is way ahead on handling the economy and terrorism.”

Like Trump, first-term Senator Cruz has sought to portray himself as a Washington outsider railing against the establishment.

On the Democratic side, Clinton remains the front-runner nationally, but she is being tested by Sanders, an independent senator and self-described democratic socialist.

She is particularly under the gun in New Hampshire, where Sanders has opened up a 14-point lead, 53 per cent to 39 per cent, according to a Monmouth University poll released yesterday.

It marks a dramatic swing from November, when the poll showed Clinton leading by three points there. Maryland’s former governor Martin O’Malley, the only other Democrat in the race, is a distant third in all polls.

New Hampshire borders Sanders’s state of Vermont and is where he would likely shine brightest in the early contests.

But the unlikely challenger is doing well in Iowa, too.

Clinton and Sanders have split the lead there in recent surveys, but a poll average compiled by RealClearPolitics.com show Sanders making steady gains in Iowa while Clinton has slumped.

The pair are now in a dead heat in Iowa, while national polls show Sanders has closed the gap to its smallest margin yet: Clinton now leads nationally by an average of 12.8 points.