Former U.S. vice president and current presidential candidate Joe Biden speaks during a campaign stop at the IBEW Local 490 in Concord, NH on June 4, 2019.

American millionaires would elect Joe Biden over President Donald Trump if the former vice president becomes the Democratic nominee, according to a new survey.

The CNBC Millionaire survey asked millionaires who they would choose for president if Trump runs against various opponents in 2020. Biden and Pete Buttigieg are the only Democratic nominees who beat Trump among the top Democratic candidates in a head-to-head race.

Fully 53% of millionaire respondents said they would vote for Biden, compared with 39% for Trump, while 9% would be undecided. In a race between Trump and Buttigieg, the mayor of South Bend, Indiana, wins 43% to Trump's 42%.

Against Sen. Bernie Sanders, Trump wins 46% to 40%. Trump beats Sen. Kamala Harris among millionaires 45% to 42%, while he beats Sen. Elizabeth Warren 47% to 40%.

Of course, millionaire voters aren't reliable predictors of election results. Millionaires backed former Secretary of State Hillary Clinton by a comfortable margin over Trump before the 2016 election, 44% to 31%, according to the CNBC Millionaire Survey.