"There's nobody I'd rather run against than little Michael," Trump said.
Bloomberg had announced in March, after spending time in early-voting states, that he was ruling out a run. He said he was "clear-eyed about the difficulty of winning" the nomination in a crowded field.
Since then, the contest has narrowed, with Sanders and Massachusetts Sen. Elizabeth Warren vying for support among the party's most progressive voters. Biden has struggled to maintain his lead with moderates, with Pete Buttigieg, the mayor of South Bend, Ind., emerging as a popular alternative.
Bloomberg has resurfaced less than three months before voting begins in the first nominating contest in Iowa on Feb. 3.
Jeff Link, a longtime Iowa Democratic strategist, jokingly referred to last weekend's big party bash in Des Moines, which drew 13 White House contestants.
"I don't remember anyone chanting, 'We need more candidates! We need more candidates!'" said Link, who is neutral in the Democratic race. "People are in the process of striking candidates off their list. Nobody wants more options."
Wolfson conceded Friday that other Democrats "already have a big head start" in Iowa and the three other earliest states to hold nominating contests, New Hampshire, Nevada and South Carolina. He suggested Bloomberg would start "on an even footing" in the Super Tuesday contests in California and other states on March 3.
Bloomberg's biggest asset is money; Forbes estimates his net worth at $52.4 billion. But even with an unlimited budget for advertising, he could still be excluded from Democratic debates if he fails to meet the increasingly high polling and donor thresholds.
The prospect of a Bloomberg candidacy was not warmly received by his rivals. Warren, who has often spoken out against billionaires using their personal wealth to finance campaigns for president, welcomed Bloomberg to the race with a tweet linking to her "calculator for the billionaires" to figure out how much they would owe under her tax on the wealth of Americans with fortunes above $50 million.
Bloomberg opposes Warren's plan: "I think the Constitution lets you impose income taxes only, so it probably is unconstitutional," he said in February in New Hampshire.