Former US Congressman Beto O'Rourke has announced that he is dropping out of the race for the Democratic nomination. 

He said that it had become clear that his campaign did not have the resources to continue to seek the party's nomination.

"My service to the country will not be as a candidate or as the nominee. Acknowledging this now is in the best interests of those in the campaign; it is in the best interests of this party as we seek to unify around a nominee; and it is in the best interests of the country," he said in a post on Twitter. 

The charismatic 47-year-old Texan entered the contest early this year with considerable buzz after narrowly losing a US Senate bid in 2018, but on the national stage he failed to ignite the enthusiasm that had propelled his statewide race.

Meanwhile, Joe Biden's grip on the 2020 Democratic nomination race has loosened in early-voting Iowa with a poll showing him slipping to fourth place.

According to The New York Times/Siena College poll, the former vice president has found himself behind leader Elizabeth Warren, Bernie Sanders and small-town mayor Pete Buttigieg.

Mr Biden, the centrist former US vice president and longtime frontrunner in the national race, also led for months in the heartland state that votes first in the battle to see who will challenge Donald Trump for the presidency next November.

But his Iowa lead has eroded since September with the rise of progressive Senator Warren, a continued solid showing by liberal Senator Bernie Sanders, and now an upswing by Mr Buttigieg.

Pete Buttigieg, mayor of South Bend, Indiana

Ms Warren leads with 22%, followed by Mr Sanders at 19%, mayor Buttigieg at 18% and Mr Biden at 17%, according to the poll.

The figures are good news for Pete Buttigieg, who has enjoyed a seven-point bump in Iowa since August, polling shows.

No other candidates in the crowded field come close in the survey of 439 Iowa Democratic caucus goers. It has a margin of error of plus or minus 4.7 percentage points.

The figures highlight Mr Biden's fade from his 28.5% support in Iowa six weeks ago, according to a RealClearPolitics poll aggregate.

Joe Biden (centre) with Bernie Sanders (L) and Elizabeth Warren (R)

Since then Mr Biden has been caught up in the scandal forming the basis of the Trump impeachment investigation, as the president stands accused of pressuring Ukraine to dig up dirt on the former vice president and his son who worked with a Ukrainian energy company.

Biden's standing in New Hampshire, which votes second in the nomination race, has also dropped, polls show, from 34.5% in mid-July to 21% now, four points behind leader Elizabeth Warren.

With pressure squeezing Biden's campaign, 14 candidates were headed to a high-profile dinner in Iowa today to woo thousands of Democratic voters.

The Liberty and Justice fundraiser is seen as a launchpad for Democratic candidates three months before the Iowa caucuses - a quirky system in which Democrats publicly announce their vote at precinct meetings statewide.

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Who Iowans choose as their back-up candidate is important, as voters whose first choice does not gain traction come under pressure to back more viable candidates.

The Times poll shows Ms Warren with the broadest appeal: she's the first or second preference of 47% of voters, compared with 34% for Sanders, 31 for Buttigieg and 28 for Biden.

Pete Buttigieg, 37, is running in the same centrist lane as Joe Biden.

Both are popular with older voters, but Mr Biden, 76, fares poorly with a younger demographic.

Only 2% of respondents under the age of 45 said they support him.