Presidential hopeful Elizabeth Warren has overtaken former vice president Joe Biden in the key state of Iowa for the first time in the race for the Democratic presidential nomination, a new poll out Sunday showed.

The Des Moines Register/CNN poll had Ms Warren with 22 per cent of the vote among likely participants in the Iowa caucuses, compared to 20 per cent for Mr Biden and 11 per cent for Bernie Sanders.

AAP

Mr Biden had until now has been the dominant frontrunner in the crowded Democratic field, but the survey showed a surge in favor Ms Warren in the state that votes first in the primary race.

Mr Biden remains the leader in national polls, but the results suggest that the race is narrowing to a two-candidate contest, as Mr Sanders loses ground to Ms Warren.

I will be damned if I do not leave behind a planet that is healthy and habitable for our kids and grandkids. — Bernie Sanders (@BernieSanders) September 22, 2019

The Massachusetts senator has vied with Mr Sanders for the progressive vote with bold ideas on health care and education while Biden has campaigned as an experienced moderate who has the best chance of defeating Donald Trump in 2020.

AAP

All other candidates polled in single digits.

Mr Biden and Mr Warren are both septuagenarians, but the former vice president, who is 76, has been the subject of more questions about his age than Warren, who is 70.