Joe Biden has thrown down the gauntlet to Michael Bloomberg, urging the billionaire to "get in and run" for the Democrat presidential nomination, and vowing to beat him.
Mr Bloomberg, 77, the former New York mayor, sent shock waves through the party this week by indicating his intention to stand, partly because of his concerns over frontrunner Mr Biden's stuttering campaign.
Many Democrats warned a Bloomberg candidacy risked a split in the party's moderate wing, between the media mogul and Mr Biden, potentially clearing the path for the liberal standard bearer, Elizabeth Warren, to become the nominee.
Mr Biden said: "Look, if he [Mr Bloomberg] wants to run, he should just get in and run. Michael's a solid guy. I have no problem with him being in the race."
But he added: "In terms of he's running because of me, the last polls I looked at, I'm pretty far ahead. If I'm not mistaken, I'm doing pretty well both relative to Trump, and relative to all the people running in the Democratic primary.
"Look at all the polls. I'm leading across the board. And, so I don't quite get this."
Mr Bloomberg filed paperwork to get his name on the ballot in Alabama, the state with the first registration deadline, on Friday.
He also rang the former Democrat governor of Iowa, Tom Vilsack, a key powerbroker in the state, which is the first to vote on Feb 3.
Following the call Mr Vilsack said: "He is in." He said Mr Bloomberg told him "it was true he is running."
Mr Bloomberg's advisers indicated his main motivation was that he believes Mr Trump, his fellow New York businessman, represented an "existential threat" to the United States.
He has previously called Mr Trump a "con" and a "dangerous demagogue."
Howard Wolfson, an adviser to Mr Bloomberg, told the Washington Post: "He woke up and said 'Oh my God, Donald Trump is going to be re-elected - I better run for president.'"
It is believed a trigger for Mr Bloomberg's late move was recent polling showing Mr Trump highly competitive in key swing states against all the current Democrat candidates, including Mr Biden.
Those results came despite Mr Trump being in the midst of an impeachment crisis.
The president also has a vastly bigger campaign war chest than all the current Democrat hopefuls. But Mr Bloomberg, America's eighth richest man with a fortune of over $50 billion, would be able to outspend him.
Anthony Scaramucci, Mr Trump's former White House communications director, said there was "no question he [Mr Bloomberg] would be the number one threat" to Mr Trump.
Mr Scaramucci said: "I think the president will be very intimidated. He [Mr Bloomberg] is a New Yorker, he can withstand the onslaught of the president's bullying. He's worth probably five to eight times the president's net worth. He can get ads up all over the country to defend himself.
"If Mike Bloomberg got the nomination it's very clear to me he would beat Trump."
However, Mr Bloomberg faced the possibility of not getting that far, and could succeed only in damaging Mr Biden while emboldening Mrs Warren.
Brad Bannon, a Democratic strategist, said: "It's lousy for Joe Biden, but great for Elizabeth Warren."
His advisers indicated Mr Bloomberg would deploy an unusual election strategy, essentially ignoring the first two states to vote, Iowa on Feb 3 and New Hampshire on Feb 11. Other candidates have already been campaigning there for months.
Instead, he would focus on "Super Tuesday" on March 3, when more than a dozen states vote on one day, blanketing them with television adverts.