Hillary Clinton has narrowly won the Democratic caucuses in Iowa, outpacing a surprisingly strong challenge from Bernie Sanders to claim the first victory in the 2016 race for president.
The former secretary of state, senator from New York and first lady edged past the Vermont senator in a race the Iowa Democratic Party called the closest in its caucus history.
The Iowa Democratic Party said that it would not do any recount of the close results, and a spokesman for the Sanders campaign said it does not intend to challenge the results of the caucuses.
The Iowa caucus-goers had to choose between Mrs Clinton's pledge to use her wealth of experience in government to bring about steady progress on party ideals and Mr Sanders's call for radical change in a system rigged against ordinary Americans. Young voters overwhelmingly backed Mr Sanders.
Mrs Clinton was bidding to banish the possibility of dual losses in Iowa and in New Hampshire, where she trails Mr Sanders, who is from neighbouring Vermont.
Mrs Clinton appeared before supporters to declare she was "breathing a big sigh of relief".
Mr Sanders had hoped to replicate Mr Obama's pathway to the presidency by using a victory in Iowa to catapult his passion and ideals of "democratic socialism" deep into the primaries.
Mr Sanders still faces an uphill battle against Mrs Clinton, who has deep ties throughout the party's establishment and a strong following among a more diverse electorate that plays a larger role in primary contests in February and March.
Iowa has long led off the state-by-state contests to choose delegates for the parties' national conventions. Historically, a victory has hardly assured the nomination, but a win or an unexpectedly strong showing can give a candidate momentum, while a poor showing can end a candidacy.
Earlier Republican Ted Cruz swept to victory over billionaire Donald Trump and Florida Senator Marco Rubio in Iowa.
Mr Cruz's victory in Monday's caucuses, which drew a record turnout, was a blow to Mr Trump, who has roiled the Republican field for months with controversial statements about women and minorities.
Mr Cruz, a fiery conservative Texas senator loathed by his own party's leaders, now heads to next Tuesday's first-in-the nation primary vote in New Hampshire as an undisputed favourite of the furthest right voters, including evangelical voters and others who prioritise an abrupt break with President Barack Obama's policies.
But Mr Trump still holds a commanding lead in New Hampshire and national polls.
New Hampshire has historically favoured more moderate candidates than Iowa, and more than 40% of the state's electorate are not registered in any political party, giving them the power to choose which parties' primary to vote in on February 9.
Mr Cruz suggested he was focused on New Hampshire but also on South Carolina, which votes 11 days later.
Mr Trump came in second slightly ahead of Mr Rubio, whose stronger-than-expected finish could help cement his status as the favourite of mainstream Republican voters who worry that Mr Cruz and Mr Trump are too extreme to win the November general election.
Mr Trump sounded humble in defeat, saying he was "honoured" by the support of Iowans. And he vowed to keep up his fight, telling cheering supporters that "we will go on to easily beat Hillary or Bernie or whoever the hell they throw up".