Michael R. Bloomberg, the former New York City mayor who joined the Democratic Party last year to crusade against President Trump, announced he would not seek the White House himself in 2020, discarding plans to mount a maverick campaign that would have tested the party’s openness to a wealthy centrist with a chameleon-like approach to partisan politics.

[Update: Michael Bloomberg expected to file for 2020 Democratic primary.]

Mr. Bloomberg’s decision appears to reflect a recognition of the long odds he would have faced as a moderate newcomer in an unapologetically liberal party, and his own unsentimental calculus about the trade-offs involved in running. After conducting polling and other research, Mr. Bloomberg’s advisers concluded he would have a real but narrow path to the nomination — and that it could all but vanish if Joseph R. Biden Jr., the former vice president, entered the race.

“I believe I would defeat Donald Trump in a general election,” Mr. Bloomberg wrote, in a Bloomberg News column announcing his decision. “But I am clear-eyed about the difficulty of winning the Democratic nomination in such a crowded field.”

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Though Mr. Bloomberg is aligned with Democrats on a range of issues like climate change and gun violence, he is also a proudly pro-business centrist in a party that has moved sharply to the left in its rhetoric and policy proposals. And his record on policing as mayor, as well as his generally favorable view of Wall Street, would likely have proven troublesome in a field that is establishing litmus tests on social justice and corporate power.