People in the US state of New Hampshire are voting in the contests to select the Republican and Democratic presidential candidates.
On the Republican side, frontrunner Donald Trump is hoping for a better performance than in last week’s Iowa caucuses, won by Senator Ted Cruz.
The main Democratic race is between Bernie Sanders and Hillary Clinton.
The tiny town of Dixville Notch cast the first votes at midnight, favouring Bernie Sanders and John Kasich.
Under New Hampshire state law, towns with populations of under 100 can apply to cast their vote as the clock strikes midnight and close the polling station as soon as everyone has voted.
Of the handful of voters in Dixville Notch in the early hours, four Democrats chose Mr Sanders, while of the Republicans two picked Donald Trump and three went for Ohio Governor John Kasich.
Mr Sanders, a senator from neighbouring Vermont and a self-proclaimed “Democratic socialist”, is hoping for a victory in New Hampshire over former Secretary of State Hillary Clinton.
Mrs Clinton, who is backed by the Democratic establishment, narrowly won in Iowa.
“For those of you who are still deciding, still shopping, I hope I can close the deal,” she said at a campaign event in Manchester on Monday.
Meanwhile Mr Sanders told cheering supporters: “We have come a long way in the last nine months. There is nothing, nothing, nothing that we cannot accomplish.”
A winter storm threatened to complicate Tuesday’s voting, bringing sub-zero temperatures and a heavy fall of snow.
The Republican race has been particularly fractious. Several candidates tore into Florida Senator Marco Rubio – who came a strong third in Iowa – during a televised debate at the weekend.
New Jersey Governor Chris Christie accused Mr Rubio of being inexperienced and scripted. “You have not been involved in a consequential decision,” he said.
Mr Rubio was also assailed by billionaire Donald Trump and former Florida Governor Jeb Bush.
On Monday Mr Trump repeated his pledge to strengthen harsh interrogation technique to terrorism suspects, vowing to bring back waterboarding and “a hell of a lot worse”.
Mr Cruz, an evangelical conservative from Texas who like Mr Trump is running on an anti-establishment platform, has called his win in the Iowa caucuses a “victory for the grassroots”.
Several of the seven Republicans on stage have staked much on New Hampshire, analysts say.
Despite its small size, the state’s place in the primary season gives it special importance as candidates try to build an early momentum.
Over the coming months each US state will pick delegates who pledge to endorse a candidate at their party’s convention in July. The victor on each side will compete in the November presidential election.