A high turnout looks likely in New Hampshire’s first-in-the-nation US presidential primary as long lines of voters queued for registration forms.
US Republican presidential hopeful Donald Trump battled to stay ahead of a charging field of his rivals in New Hampshire and Democratic candidate Bernie Sanders sought a big win over Hillary Clinton.
Mr Trump and Mr Sanders were favoured to win their races, riding a wave of voter anger at traditional politicians seeking their parties' presidential nomination.
Despite being an overwhelming front-runner, Mrs Clinton's razor-thin victory in Iowa made her look vulnerable and a double-digit loss to Mr Sanders in New Hampshire would add pressure on her to improve her performance.
There was some talk among Clinton loyalists of a possible campaign shakeup.
Mr Trump was under pressure to deliver a victory after he was beaten in the first nominating contest - the Iowa caucuses on 1 February - by Texas Senator Ted Cruz despite having had a big lead in pre-caucus polls.
The billionaire businessman posted a video on Facebook urging his supporters to vote.
"The polls don't mean anything if you don't get up, don't get out, don't vote," he said. "We have to vote. You have to make this change."
New Hampshire is the second state in the process of picking party nominees for the 8 November election to replace President Barack Obama.
The polls were to close at 7pm EST (12am Irish time)and New Hampshire officials predicted a historic high turnout of about 550,000.
President Obama, who has not yet endorsed a candidate from among his fellow Democrats, expressed surprise at the leads in polls held by Mr Trump and Mr Sanders.
"Early on, often times, voters want to just vent and vote their passions," he said.
Primary votes were already counted in Dixville Notch, a town of about a dozen people that prides itself on being the first in the state to vote.
Mr Sanders won all four Democratic votes there while in the Republican race Ohio Governor John Kasich beat Mr Trump 3-2.