Joe Biden has an Elizabeth Warren problem.

The big picture: Warren is rising in national, battleground and state polls, electrifying liberals with her very specific and very liberal ideas — and she's well-positioned to undermine Biden, win or lose.

Warren has the ability to unify the progressive wing of the Democratic Party, especially if she locks arms with AOC in attacking Biden’s more cautious and conventional politics.

the ability to unify the progressive wing of the Democratic Party, especially if she locks arms with AOC in attacking Biden’s more cautious and conventional politics. Even if she tops out, her politics are much closer to Bernie Sanders or Kamala Harris, so she could help lift another candidate over Biden with her endorsement and support.

Be smart: Biden’s people are worried about her rise, and feel they caught a break in not having to face her in the first debate.

"Avoided her and thus avoided a potential first round TKO," a Biden confidant told me.

The intrigue: President Trump has told people he thought he killed her campaign with his "Pocahontas" smear and that he’s surprised by her resurgence, Axios' Jonathan Swan reports.

Trump, in private chats, wonders if she he sprang the taunts too early.

A Biden source tells me: "What we see here is a battle between Bernie and Warren for the left lane of the Dem electorate — Warren’s rise is largely coming from Bernie, not from Biden. A good reminder that Biden appeals to a much wider swath of the electorate than either of them do."

A Biden adviser adds:

"Warren's 'rise' isn’t a surprise. ... What was surprising was Warren's slow start — she’s now where we thought she'd be upon entry."

isn’t a surprise. ... What was surprising was Warren's slow start — she’s now where we thought she'd be upon entry." "It doesn't matter to us whether it's Bernie or Warren — either way we are happy to have someone who is operating in the far left lane in second place."

Go deeper: Elizabeth Warren on the issues, in under 500 words