Bernie Sanders and Donald Trump win New Hampshire primary

US Senator Bernie Sanders has won the Democratic presidential primary in the key state of New Hampshire, giving a big boost to his campaign for the 2020 White House bid as the race to his party's nomination gathers momentum over the next few months.

President Donald Trump, 73, who is seeking his reelection, won the New Hampshire Republican primary with a massive vote.

Sanders, 78, who lost to former secretary of state Hillary Clinton in the 2016 presidential primaries in New Hampshire, was ahead of his closest rival Pete Buttigieg.

With more than 85 per cent of the votes counted, the Senator from Vermont took 26 per cent support, followed by Buttigieg closely with 24.4 per cent and Amy Klobuchar coming third with 19.8 per cent of the votes.

Massachusetts Senator Elizabeth Warren and former Vice President Joe Biden were headed for distant fourth and fifth spots respectively.

"Now that we've gained another victory in New Hampshire, the establishment and financial elite are going to throw everything at us except the kitchen sink. And I have a feeling that the kitchen sink is coming pretty soon as well," Sanders said after his victory in New Hampshire.

The Republican party and the Trump campaign said that Sanders winning the Democratic primary reflects that socialism has gained momentum in the Democratic party.

"One thing that is clear: Bernie Sanders winning his second state in a row shows how Socialism is now the mainstream in today's Democrat Party," Republican National Committee (RNC) Chairwoman Ronna McDaniel said in a statement.

"The Democrat story in New Hampshire is the continued dominance of big government socialist policies and the success of their standard bearer, Bernie Sanders," said Brad Parscale, Trump 2020 campaign manager.

"No matter which Democrat eventually emerges from their months-long dumpster fire of a primary process, we know the contrast will be President Trump's record of accomplishment and optimistic view of the future versus Democrats and their socialist, job-killing agenda," he added.

Ahead of the US Presidential election later this year, White House hopefuls are contesting for their party's nomination in primary elections, known as primaries, across the country.