Donald Trump is facing his last remaining Republican opponent, Nikki Haley, in a primary election in New Hampshire on Tuesday.

The former US president is hoping to deliver a knockout blow to the ex-South Carolina governor's campaign to be the Republican nominee.

Ms Haley hopes New Hampshire's bloc of independent voters will help her pull off an upset victory.

The contest follows Mr Trump's landslide win last week in Iowa.

The eventual Republican nominee will challenge the Democrat nominee, likely President Joe Biden, in November's general election.

A contest between Mr Biden, 81, and Mr Trump, 77, would be a re-run of the 2020 presidential vote.

Democrats are also voting in their primary in New Hampshire on Tuesday, but because of a dispute between national and local party officials, Mr Biden is the first sitting president not to appear on the ballot.

The race for the Republican nomination narrowed to just two on Sunday, after Florida Governor Ron DeSantis came a distant second in Iowa's caucuses and then quit and endorsed Mr Trump.

Mr Trump already held a double-digit lead in many opinion polls taken in New Hampshire before Mr DeSantis' exit. Most surveys suggest a majority of the Florida governor's supporters view Mr Trump as the next best option.

A new poll released by Suffolk University and the Boston Globe on Tuesday morning showed Mr Trump 19 points ahead of Ms Haley, 57% to 38%. A second poll from Monmouth University and the Washington Post showed him with an 18 point lead, 52% to 34%.

At a rally in Laconia, New Hampshire, on Monday night Mr Trump said that his campaign has "to win by big margins" to "send a signal that we're not playing games. This country has gone to hell."

"If you want a losing candidate who puts America last, vote for Nikki Haley," he added.

He told the crowd that "globalists and radical left communists" were convincing liberals to vote for her in the Republican primary.

"They want to run against her because she's so easy to beat," he added.

Most voters in New Hampshire will cast their ballots during the day on Tuesday but in tiny Dixville Notch, a handful of people voted at midnight in a decades-old tradition. All six registered voters in the resort town chose Nikki Haley over Donald Trump.

At a campaign event earlier in the day, Ms Haley, 52, said Mr Trump was preoccupied with the criminal and civil cases he faces. He denies all charges, claiming political persecution.

"When you hear Trump speak," she said, "what's he talking about? Grievances, the past. He's talking about vendettas.

Ms Haley, who was UN ambassador under President Trump, added: "When you go out on Tuesday, you're going to decide: do you want more of the same, or do you want something new?"

Speaking at a separate event in Salem on Monday night, Ms Haley urged voters to support her, arguing that polls show she has a better chance in a general election against President Biden.

"We have a country to save and we have the opportunity to save her," she said.

Anyone in New Hampshire can vote in the primary, and the state is seen as more moderate than deeply conservative Iowa. Mr Trump won the Republican nomination in the state in 2016 and 2020.

The Haley campaign is pinning its hopes for a potential shock win on the 40% of voters in New Hampshire who are registered independent.

The Democratic New Hampshire primary, which is also held on Tuesday, is at the centre of a dispute between state party officials and the national Democratic National Committee.

New Hampshire is usually the first primary in the US election calendar, but the national DNC changed the order to make South Carolina first, arguing that it better reflected the country's diversity.

Local party officials in New Hampshire decided to run the primary anyway, so the DNC said it would not award any delegates, effectively not recognising the results, and Mr Biden would not be on the ballot.

Instead, some of Mr Biden's Democratic allies in New Hampshire are urging voters to write his name on the ballot when they vote.

Polls in the state will begin closing at 1900 EST (0000 GMT), with the last closing an hour later. Preliminary results are expected around this time, although the final tally may take several more hours.