NEW YORK (Reuters) - Former Vice President Joe Biden expanded his lead over more than a dozen other candidates for the Democratic presidential nomination this month as U.S. House Democrats looked into allegations that President Donald Trump tried to pressure a foreign leader to investigate him, according to a Reuters/Ipsos poll released on Friday.

The Oct. 17-18 opinion poll found that 21% of Democrats and independents said they would vote for Biden in statewide nominating contests that begin next year, up 3 percentage points from a similar poll that was conducted at the end of September.

Democrats in the House of Representatives are holding hearings to investigate whether Trump improperly pressured Ukraine to investigate Biden and his son Hunter, who sat on the board of a Ukrainian energy company.

The House could vote to impeach Trump later this year, which would trigger a trial in the Republican-controlled Senate where a conviction and ouster seem unlikely.

Trump denies he did anything wrong.

So far, the inquiry does not appear to have shaken up public support for Biden or the other candidates for the Democratic nomination.

According to the poll, 16% of Democrats and independents said they would support U.S. Senator Bernie Sanders of Vermont and 15% said they would back U.S. Senator Elizabeth Warren of Massachusetts.

Mayor Pete Buttigieg of South Bend, Indiana, received about 5% support in the poll, and U.S. Senator Kamala Harris and former Texas congressman Beto O’Rourke both received 3%.

With less than four months to go before Iowa holds the first nomination contest, the race for the Democratic nomination remains wide open. About 1 in 5 said they remain undecided, and nearly two-thirds of Democrats and independents said they could still change their minds.

Many of the candidates, including Buttigieg, O’Rourke and Harris, are still relatively unknown to a majority of Americans, and only about 1 in 10 said they watched this week’s Democratic debate in Ohio.

The Reuters/Ipsos poll was conducted online, in English, throughout the United States. It gathered responses from 1,116 adults in all, including 703 Democrats and independents. It has a credibility interval, a measure of precision, of about 4 percentage points.