Washington (AFP) - As the two dozen Democrats aspiring to the US presidency struggle for attention, a new survey in the first state to vote in 2020 shows five of them, led by former vice president Joe Biden, emerging with early leads.

The CNN/Des Moines Register/Mediacom poll of Iowa voters comes a full eight months before they convene in caucuses to pick a favorite, but it carries weight since Iowa's early voting status helps shape voter impressions nationwide.

The survey shows 24 percent of Iowa voters favoring Biden, giving him a substantial lead over Senator Bernie Sanders of Vermont, at 16 percent.

Virtually tied with Sanders are Senator Elizabeth Warren of Massachusetts (15 percent) and South Bend, Indiana Mayor Pete Buttigieg (14 percent).

The only other Democrat polling above 5 percent is California Senator Kamala Harris (7 percent).

After Harris, support drops sharply, with former Texas congressman Beto O'Rourke and Minnesota Senator Amy Klobuchar polling at 2 percent.

All other candidates registered 1 percent or less. The survey had a sampling error of 4.0 percent.

Both Sanders and O'Rourke emphasized in television appearances Sunday how early it is in the nominating process.

Sanders noted on CNN that when he ran against Hillary Clinton for the nomination in 2016, both received about half the Iowa vote. Now, given the size of the field, "I don't think anybody will."

But he insisted he had a "very strong chance" of being picked to run against "the worst president in the modern history of this country, Donald Trump."

The first televised Democratic debates are set for June 26 and June 27 in Miami.

To qualify to take part, Democrats must register 1 percent or higher in major national polls or surveys from early voting states like Iowa. Alternatively, they must have received donations from 65,000 donors.

Most of the Democrats, but not Biden, will appear Sunday night in Iowa at a major party fundraiser in Cedar Rapids.

A Trump rally in Iowa is set for Tuesday.