Mr. Bloomberg has done more than just give. Last week, he traveled to Florida to campaign for the Democratic nominee for governor there, Andrew Gillum, at a Jewish center in Pembroke Pines. He donated to Mr. Gillum; his anti-gun group, Everytown for Gun Safety, also donated; and he hosted a fund-raiser, too.

This Tuesday, back in New York, Mr. Bloomberg hosted a fund-raiser for Phil Bredesen, the former Democratic governor of Tennessee and current candidate for United States Senate.

Mr. Bloomberg is not solely backing Democrats this year; in June, he held a fund-raiser for Representative Peter King, Republican of New York, with whom he worked closely following the terrorist attacks of Sept. 11, 2001.

Still, his re-registration as a Democrat is not a shock.

Mr. Bloomberg spoke at the 2016 Democratic convention in favor of Hillary Clinton (“The richest thing about Donald Trump is his hypocrisy,” Mr. Bloomberg, a billionaire, said then.). And he has begun flirting with the presidency himself in recent months, he told The New York Times in September. “That’s not to say I’m with the Democratic Party on everything, but I don’t see how you could possibly run as a Republican. So if you ran, yeah, you’d have to run as a Democrat.”

In that interview, Mr. Bloomberg hinted that he might formally register as a Democrat, saying he would “look at it” but had no imminent plans.

“I think at the moment, there’s no reason to do anything,” Mr. Bloomberg said then, of changing his registration. “But if I think — if I wanted to change parties and I thought it sent the right message, I would do it.”

Mr. Bloomberg’s registration announcement comes ahead of New York’s deadline later this week and after the deadline has passed in more than a dozen states in recent days.