Bernie Sanders has a clear lead over Hillary Clinton as the two Democratic Party candidates head into Tuesday's primary in the north-eastern US state of New Hampshire.

The 74-year-old senator from the neighbouring state of Vermont, who is running a decidedly left-of-centre campaign by US standards, has a local advantage over Clinton, a former secretary of state and senator for New York state.

For the Republicans, controversial billionaire Donald Trump leads Marco Rubio, the Florida senator who had a surprisingly strong third place finish in the Iowa caucuses that kicked off the selection process February 1.

Clinton edged out Sanders in Iowa by just 0.2 of a percentage point among the Democrats, with former Maryland governor Martin O'Malley far behind in third place.

Conservative Texas senator Senator Ted Cruz won the Republican contest in Iowa with 27.6 per cent, ahead of Trump on 24.3 and Rubio on 23.1 per cent.

Trump, who has broken from standard political conventions with his strident anti-immigrant rhetoric, had led in the polls ahead of the election.

The early primaries to choose respective candidates for the November 8 presidential election are seen as a kind of trial for the presidential hopefuls.

The political landscape of both parties is littered with the corpses of candidates previously seen as possible presidents, as author Elaine C Kamarck writes in her book Primary Politics.

These candidates fell in the early primaries and caucuses in the smaller states, either by failing to win or by failing to contest them. This year has been no exception, as many have already fallen by the wayside.

The New Hampshire primaries are organized at the state government level, not by the parties, as was the case in Iowa. Voting is by secret ballot in 307 voting stations.

The state's population is 95 per cent white and does not accurately represent the demographics of the US as a whole.

One New Hampshire peculiarity is that the state does not levy income tax or value-added tax. There is also a strong libertarian movement that favours slimmed down government and deregulation, with strong protection for individual privacy.

All registered voters may participate in the primary, irrespective of whether they are registered as Democrats or Republicans.

Those not belonging to either party are considered independents and may vote for either the Democrat or Republican candidate of their choice.

Current figures put the number of New Hampshire independents at 389,472, while there are 231,376 Democrats and 262,111 Republicans.

New Hampshire sends the relatively small number of 32 delegates to the Democratic Party convention and 23 to the Republican. Initial results from the primaries are expected at 11 pm local time Tuesday (0400 GMT).

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