By Veronica Stracqualursi, CNN

(CNN) -- Former Vice President Joe Biden and Sen. Kamala Harris are the two Democratic candidates leading the large field of potential 2020 challengers to President Donald Trump, according to a new Washington Post-ABC News poll.

However, a majority of Democrats and Democratic-leaning voters remain undecided about which candidate they want as their party's nominee to take on Trump, whom a majority will "definitely not vote for" if he's up for re-election, the Post-ABC poll out Tuesday showed.

When asked the open-ended question of whom they'd support in a primary or caucus today, Biden, who has not yet announced whether he's running in 2020, was the person most named by 9% of Democratic-leaning voters. Harris, a Democratic senator from California, was named by 8%.

Forty-three percent of Democrats and Democratic-leaning voters said they have no opinion. Another 7% said no one or none of the current candidates, 4% said anyone, and 1% said someone new.

The poll was conducted before Harris officially launched her 2020 presidential campaign on Sunday.

As there is no clear Democratic frontrunner, the Post-ABC poll also shows reelection troubles for Trump.

Fifty-six percent of all Americans said they would "definitely not vote for" Trump should he be the 2020 Republican nominee.

One in four Republicans and Republican-leaning voters approve of Trump's job performance, yet nearly 1 in 3 say they'd like the Republican Party to nominate someone other than Trump to be its 2020 candidate for president.

Even though he's a Republican, Trump was mentioned by 4% of Democrats and Democratic-leaning voters as the candidate they'd support today. Independent Sen. Bernie Sanders of Vermont also got 4%.

Former Texas Rep. Beto O'Rourke, who lost a bid for Senate this past election, was named by 3%.

Others named included former first lady Michelle Obama, who has said she's not running, and Sen. Elizabeth Warren of Massachusetts, who has announced a presidential exploratory committee, at 2%.

Sen. Cory Booker, Montana Gov. Steve Bullock, former Secretary of State Hillary Clinton, Sen. Amy Klobuchar, House Speaker Nancy Pelosi and Oprah Winfrey were all each mentioned by 1% of Democratic-leaning voters.

The Post-ABC poll was conducted January 21-24 among 1,001 adults, with an overall margin of error of plus or minus 3.5 percentage points. The margin of error is plus or minus 5.5 points among the 447 Democrats and Democratic-leaning independents and the 405 Republicans and Republican leaning-independents. Voters were reached via cell phone or landline.