Former US Vice President Joe Biden has topped a poll of Iowa voters that also shows Senator Bernie Sanders gaining momentum against him in the number two spot.

Biden, who has not announced whether he is running in the 2020 election, is the first choice for president of likely Iowa Democratic caucus-goers with 27 per cent in the Des Moines Register/CNN/Mediacom Iowa Poll. Sanders, 77, got 25 per cent.

"If I'm Joe Biden sitting on the fence and I see this poll, this might make me want to jump in," J Ann Selzer, president of Selzer & Co, which conducted the poll, told the Des Moines Register on Sunday.

The newspaper's Iowa poll has a long track record of relative accuracy in the state that kicks off the presidential nominating process. In this cycle, Iowa will hold the first contest in the Democratic race in February 2020.

Nearly 65 per cent of the voters said Biden, 76, who was also a US senator first elected in 1972, has more experience than any other candidate and should enter the race, while 31 per cent said his time as a candidate has passed.

Sanders, a progressive populist who held a rally in Iowa as the poll was being conducted last week, gained 6 percentage points from 19 per cent in the group's previous poll released in December. Biden fell five percentage points from 32 per cent in the last poll.

At least a dozen major candidates already have jumped into the Democratic contest to pick a nominee to challenge Republican President Donald Trump, and Democrats are still waiting for decisions in coming weeks from other big names such as Biden and former Congressman Beto O'Rourke of Texas.

In most national polls of Democrats, Biden has a solid lead while Sanders, who lost the 2016 Democratic nomination to Hillary Clinton, typically is in second.