Voters in a tiny northern US community have cast the first ballots in the first presidential primary, handing victory to John Kasich for the Republicans and Bernie Sanders for the Democrats.

The polling station in Dixville Notch, New Hampshire, opened at midnight, allowing nine registered voters to cast their ballots hours before most of the state on Tuesday.

It went 3-2 to Kasich over Donald Trump for the Republicans, and 4-0 for Sanders over former US secretary of state Hillary Clinton for the Democrats, the Washington Times reported.

The result is sometimes treated as a predictor of how the rest of the state will vote.

Dixville Notch has kept the tradition of midnight voting alive continuously since 1960, according to USA Today, and this year was joined by two other nearby communities, Millsfield and Hart's Location, also with just a handful of voters each.

Polling stations elsewhere in the state open at 6am.

Voting will continue until 7pm or 8pm in different locations.

Voters in the northeastern US state are holding the second ballot in the long process of electing the US president, and the first to be held under the procedure of a state primary.

The first ballot was by caucus on February 1 in Iowa, where US Senator Ted Cruz finished on top among Republicans and Clinton beat Sanders, a US senator, by a whisker among Democrats.

The spotlight has been on New Hampshire since the Iowa vote, and the candidates spent the last day before the primary campaigning for support. Nearly all the Republicans made appearances on Monday.

Polls showed the left-wing Sanders, who represents the neighbouring state of Vermont, far ahead of Clinton for the Democrats, while billionaire Trump holds a large lead among Republicans across the state as a whole.

Trump also held a lead in the polls going into the Iowa caucuses, but finished second to arch-conservative Cruz.

Marco Rubio, the third-place finisher in Iowa, has emerged as a compromise candidate who is less polarising than Trump and Cruz.

The US senator from Florida is looking to build on the momentum he had coming out of Iowa, but Ohio Governor John Kasich and New Jersey Governor Chris Christie have strong organisations in the state and are expected to do better than they did in Iowa, where neither made much of a showing.

Former Florida governor Jeb Bush also hopes to do well in New Hampshire. His candidacy has not taken off, but most observers said he has honed his message. He performed well in a debate on Saturday.

The New Hampshire primary is different from Iowa in that it is organised not by the political parties, but by the state. There are 307 voting stations and voting takes place by secret ballot.

All registered voters - not just registered Democrats and Republicans - are allowed to participate in the primary. Unaffiliated voters are classified as independents and may vote for candidates in either party.

Independents make up about 43 per cent of the New Hampshire electorate, while Democrats and Republicans make up about 30 per cent each.