Joe Biden's grip on the 2020 Democratic nomination race loosened Friday in early-voting Iowa with a poll showing him slipping to fourth place behind leader Elizabeth Warren, and overtaken by small-town mayor Pete Buttigieg. 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 Buttigieg. The New York Times/Siena College poll showed a tight top-tier race as candidates gear up for next February's closely watched Iowa caucuses.

Warren leads with 22 percent, followed by Sanders at 19 percent, Buttigieg at 18 percent and Biden at 17 percent, according to the poll. The figures are good news for 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 Biden's fade from his 28.5 percent support in Iowa six weeks ago, according to a RealClearPolitics poll aggregate. Since then 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 Biden 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 percent in mid-July to 21 percent now, four points behind leader Warren. With pressure squeezing Biden's campaign, 14 candidates were headed to a high-profile dinner Friday in Iowa to woo thousands of Democratic voters.