Washington (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.

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