Hillary Clinton has won the Democratic caucuses in Iowa, beating back a 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 Tuesday 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.

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Democratic presidential candidate Bernie Sanders poses for photos during a caucus night rally on Monday, Feb. 1, 2016, in Des Moines, Iowa. (Evan Vucci/Associated Press)

Democrats form groups at caucus sites, publicly declaring their support for a candidate. The final numbers are awarded proportionately, based on statewide and congressional district voting, determining Iowa's 44 delegates to the national convention.

Clinton had been expected to cruise to victory in Iowa and beyond. But Sanders has appealed to the Democrats' liberal base, especially the young, who are concerned about growing income inequality and the shrinking of the middle class.

Although Clinton said she was "breathing a big sigh of relief," and her campaign said it had won an outright victory, the neck-and-neck contest was a blow, evoking the setback she faced in 2008 after her upset loss to then senator Obama. Given the closeness of the Democratic caucuses, the AP did not declare a winner.

Eight years ago, Clinton's victory in New Hampshire breathed fresh life into her campaign. But New Hampshire is also familiar territory for Sanders, who represents neighbouring Vermont in the Senate and is well-known among the state's voters. Sanders' sizable lead over Clinton in New Hampshire polls has held steady or increased in recent weeks.

Clinton had a big advantage with voters 65 and over, according to polling data, with Sanders able to boast of an even wider disparity — at over 80 per cent support — from voters 30 and under.

Clinton's victory in Iowa means she will collect 23 delegates and Sanders will win 21. With her advantage in superdelegates — the party officials who can support the candidate of their choice — Clinton now has a total of 385 delegates. Sanders has 29.

It takes 2,382 delegates to win the Democratic nomination for president.

Cruz swept to victory

 Meanwhile, Republicans and Democrats descended on the tiny New England state of New Hampshire, which votes next Tuesday.

On the Republican Iowa caucuses, Texas Senator Ted Cruz, a fiery conservative loathed by his own party's leaders, swept to victory on Monday, overcoming billionaire Donald Trump and a stronger-than-expected showing by Florida Senator Marco Rubio.

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On the Republican Iowa caucuses, Texas Senator Ted Cruz, a fiery conservative loathed by his own party's leaders, swept to victory on Monday. (Charlie Neibergall/Associated Press)

The outcome in the country's first nominating contest drew a line under voter dissatisfaction, especially among Republicans, with the way government in Washington operates, with anger over growing income inequality and fears of global turmoil and terrorism.

Cruz's victory in Monday's caucuses, which drew a record turnout, was a blow to Trump, the real estate mogul who has roiled the Republican field for months with controversial statements about women and minorities.

But 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 per cent of the state's electorate are not registered in any political party.

Cruz on Tuesday suggested he was focused on New Hampshire but also on South Carolina, which votes 11 days later.

Trump came in second slightly ahead of Rubio, whose stronger-than-expected finish could help cement his status as the favourite of mainstream Republican voters who worry that Cruz and Trump are too caustic to win the November general election.

Trump 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."

Some of the establishment Republican candidates have been focusing more on New Hampshire than Iowa, including former Florida governor Jeb Bush, Ohio Gov. John Kasich and New Jersey Gov. Chris Christie.

The caucuses marked the end of at least two candidates' White House hopes. Former Maryland governor Martin O'Malley ended his longshot bid for the Democratic nomination, and former Arkansas governor Mike Huckabee dropped out of the Republican race.