After failing to live up to expectations in Iowa this week, Donald Trump aims to get his campaign for the Republican presidential nomination back on track in New Hampshire on Tuesday.

Trump leads in the polls, but they also had him ahead in Iowa. And now the momentum has swung to his two biggest opponents.

Meanwhile, on the Democratic side, Hillary Clinton seems poised to lose New Hampshire after only narrowly defeating Bernie Sanders in Iowa.

The latest polls of likely voters in New Hampshire suggest a robust lead for Trump, a businessman and reality television star. On Thursday, FiveThirtyEight.com calculated Trump's average support in the polls to stand at 33 per cent, followed at a distance by Florida Senator Marco Rubio at 13 per cent, Texas Senator Ted Cruz at 12 per cent, Ohio Gov. John Kasich at 11 per cent and former Florida governor Jeb Bush at 10 per cent.

Polling in New Hampshire has been relatively consistent, with Trump scoring in the high-20s to mid-30s and the other candidates registering somewhere in the teens. That lead of roughly 20 points for Trump is far larger than the one he enjoyed in Iowa, which stood at between five and seven points.

But primaries can move quickly, and polls have yet to fully record the effect of Cruz's victory in Iowa, where he beat Trump with 28 to 24 per cent of the vote.

And Rubio's third-place finish of 23 per cent, which was a significantly better performance than expected, has also not fully registered. However, the few polls taken after Iowa have suggested a modest uptick in support for the Florida senator.

Marco Rubio is looking to build his momentum in New Hampshire. (Pete Marovich/Getty Images)

Sanders, a Democratic senator from Vermont, enjoys a similarly comfortable lead in the polls. FiveThirtyEight.com pegged his support at 55 per cent, against just 38 per cent for Clinton. The few post-Iowa polls that have been conducted show no significant change.

That Sanders is on track to win New Hampshire should come as little surprise — few states are demographically more favourable for him. It also neighbours the state Sanders represents in the U.S. Senate.

But the path will get more difficult for Sanders after New Hampshire, as the primaries move to states that lean heavily toward Clinton, who still leads Sanders by a wide margin in national polls.

Live free or die (or finish in the top 2)

New Hampshire is the scene of the first primary of the U.S. presidential campaign, which will come to a close in November after the Republicans and Democrats settle on their nominees this summer (Iowa, which voted this week, held a caucus rather than a primary). And like Iowa, it is both unrepresentative of the rest of the United States and demographically insignificant, ranking 41st in terms of its population.

Nevertheless, the state carries an out-sized weight in the primaries because it comes so early in the campaign. Its history of selecting each party's eventual nominee is mixed, but since 1972 no candidate has won his party's nomination without placing first or second in New Hampshire.

Today on the Pollcast, we’re going States Side. Primary season is on in the United States and to help us break down what’s going on, we called Harry Enten. He’s a senior political writer and analyst for FiveThirtyEight.com. 17:26

With only two candidates left in the race after former Maryland governor Martin O'Malley dropped out, that streak will continue for the Democrats. But it is not clear that the Republicans — who have also seen their slate winnowed down following the withdrawals of Rick Santorum, Mike Huckabee and Rand Paul — will also keep the tradition up, with five candidates polling in double-digits.

Nothing's for granted in the Granite State

With the primary a few days away and voters and the media still digesting the results of the Iowa caucuses, there is still plenty of scope for things to change between now and Tuesday.

The experience of Iowa suggests that pollsters will need to keep in the field up to the last moment in order to get the best bearing of the race. The polls that did so in the Midwestern state came closest to the mark, capturing elements of both Trump's sagging campaign and Rubio's surge. The debates scheduled over the weekend may also have an effect, though the Super Bowl on Sunday will make it more difficult for pollsters to reach voters.

The next few days may be decisive. Though Rubio finished in third place in Iowa, his results beat expectations by a significant margin. He can now use his performance in Iowa to claim that he is the "safe" candidate for Republicans to rally around, compared with the volatile Trump and ultra-conservative Cruz, who are both running campaigns that are aimed squarely against the establishment of the Republican Party as much as the Democrats.

Already, Rubio is starting to gather more endorsements from other elected Republicans, surpassing the previous "establishment" favourite Bush.

Vermont Senator Bernie Sanders is favoured over Hillary Clinton in New Hampshire. (Steve Marcus/Reuters, Mic Smith/Associated Press)

Iowa has less to say about the Democratic tilt, though Sanders did show he has the ability to turn grassroots support into real votes. But Clinton's narrow margin of victory (0.3 percentage points) may have had more to do with O'Malley than anything the pollsters missed. Because of the Democrats' caucus system in Iowa, O'Malley failed to get on the final ballot in most precincts. Anecdotal evidence suggests that many O'Malley supporters went to Sanders instead of Clinton.

But with Sanders the heavy favourite in New Hampshire, all eyes will be on the Republican primary. Each of the major candidates has a lot riding on the result.

This may be the last opportunity for Kasich and Bush to show they deserve to still be in the race.

Cruz needs to demonstrate that Iowa was not just a fluke delivered to him by evangelicals, who disproportionately make up the Republican electorate in the state.

Expectations for these candidates, however, remain low.

Rubio's campaign to be the consensus candidate will be seriously tested in New Hampshire if he doesn't come out of the state with another strong performance. And Trump's claim to be a winner, already dented by Iowa, could be in serious danger.

If Trump doesn't win in New Hampshire, his presidential bid may make him what he despises most — a loser.