Former Vice President Joe Biden is inching closer to a run for the White House. CBS News has learned that he's started to tell potential donors about his plans for the 2020 presidential election. After nearly accidentally announcing his plans in a weekend speech, several people familiar with his plans tell CBS News that Biden has started phoning potential donors this week who can help him raise millions of dollars, fast.

Part of Biden's challenge is that unlike Bernie Sanders, Beto O'Rourke or Kamala Harris, he's never had a vast national network of supporters willing to donate money online. So it's unclear if he will be able to raise big money on day one.

So far, the number Biden needs to beat is $6.1 million -- that's the amount O'Rourke raised in the first 24 hours of his presidential campaign. Sanders raised about $5.9 on day one.

Early polling shows Biden is at the top of an already crowded field of 14 candidates running for the 2020 Democratic nomination. And Biden's former colleagues, including Republican Senator Jeff Flake and Democratic Senator Chris Coons, say a Biden run would pose the greatest challenge to President Trump in 2020.

"He can speak to those states that President Trump won, the Rust Belt in particular, and he's seen as more of a centrist. He was a senator for 30 some years. He knows how to work with the Senate, with the Congress, and I think that's certainly one that worries Republicans," Flake told CBS This Morning this month.

"Joe Biden is someone who sees the differences in our country and inspires us, reminds us of the ways in which we have overcome them in the past and will lift us up," Coons said.

While Biden can easily attract support in suburban swing districts and Midwestern states key to previous Democratic presidential victories, the question remains if he will be able to attract younger and minority support that's currently fueling many of his would-be Democratic challengers

But new polling by the University of Chicago's GenForward Survey Project shows that Biden currently leads declared and potential Democratic presidential candidates in support among millennial voters.

The poll found that 21 percent of millennials back Biden over Democrats who have entered the primary field, including Sanders, Harris, Cory Booker, Amy Klobuchar and Elizabeth Warren. Biden also enjoys strong support among young minority voters -- with support from 29 percent of Asian American millennials and 28 percent of African American millennials.

As Biden starts phoning donors, top aides are starting to interview potential staffers for roles on the campaign. All signs still point to an announcement at some point in mid-April