Oddly, some voters have already cast their ballot almost 18 hours ago, in the middle of the night.

Quirks of the New Hampshire electoral system mean that places with a particularly low number of residents can vote when they like. So the residents of three hamlets - including Dixville Notch, near the Canadian border - voted at midnight, as is traditional.

Image source, AFP

Until recently, Dixville Notch had only four residents, which would not have been enough for it to vote at this time. Then one resident decided to move back, and everything was OK again.

So who came out on top there? It was Michael Bloomberg, who was not even on the ballot, until the people of Dixville Notch decided to add him.

In New Hampshire, people can register for a particular party even on election day in order to cast a vote.

This could change the outcome - CBS News, the BBC's US partner, reports that almost half of New Hampshire voters, 48%, didn't decide who they were voting for tonight until the past few days.

And nearly half also said the last Democratic debate was a key factor in their choice. (Read our North America reporter Anthony Zurcher's analysis of that debate here.)

For context, in 2016, only a quarter of voters said the same in primary exit polls.

As for what these voters want in their next president? Nearly 40% want a shift to even more liberal policies while another 40% think things should return to Obama-era ways.

Unsurprisingly, most Democrats in the northeastern state are unhappy with Trump - and 81% describe themselves as angry with his administration.

And as a result, 62% of these early voters say they prefer a candidate who can beat the Republican president over the one they might agree with most.

Quincy is all in for Bernie Sanders in New Hampshire, and that's put the 23-year-old at odds with her father, Ron. He's backing Elizabeth Warren in New Hampshire's Democratic primary: the BBC's Hannah Long-Higgins met them.

This kind of generational battle between ideology and pragmatism is nothing new - but it could be a key factor as Democrats decide who has the best chance of beating Donald Trump in November's presidential election.

Jane O'Brien
BBC News, New Hampshire

Ask somebody in New Hampshire what they think of any particular candidate and they’ll reply jokingly: “I don’t know - I’ve only met them twice.”

As a New Hampshire resident I can now say the same.

I’ve interviewed Senator Michael Bennet at a café in Concord and Senator Amy Klobuchar at the offices of the Conway Daily Sun. I’ve stood just a few feet away from former Vice President Joe Biden, his security detail nowhere in sight, and shivered with Bernie Sanders in an icy car park. I’ve sat with voters in bars and cafés, listened to their hopes and fears and marveled at how much they know about politics. Most of them could put TV pundits to shame.

Image source, Reuters

For the last 12 years in Washington DC I have spent much of my time on Capitol Hill interviewing lawmakers of both parties and reporting on the White House. Living in New Hampshire makes me feel much more like an insider as here politics are personal and candidates are stripped bare of the trappings of power.

There’s been a lot of debate about whether New Hampshire deserves to be the first primary in the nation, and it’s a valid question; other states are certainly more diverse and arguably more representative of American society. But as the results come tonight, I will feel truly part of the process.

Eight days since the chaotic, glitch-ridden, fraught Iowa caucuses, it's the turn of voters in New Hampshire to pick which Democrat they think should run against Donald Trump in the 2020 presidential election.

We're still months away from knowing for definite who that will be. But in Iowa, left-wing firebrand Bernie Sanders and Pete Buttigieg, the former mayor of South Bend, Indiana, got their noses just in front.

Both could do well again in New Hampshire, a small north-eastern state of 1.3m people. But others such as Massachusetts senator Elizabeth Warren and Joe Biden will be hoping to do better than last week.

Stick with us over the next few hours - we'll be bringing you all the latest from our teams in New Hampshire and here in Washington DC.