Much like Mr. Biden, Mr. Bloomberg is wagering that Democratic voters will care far more about defeating Mr. Trump than any other political or ideological consideration.

Mr. Bloomberg’s message, at the start, is leaning heavily into his biography as a business executive and as the mayor of New York City starting in the aftermath of the Sept. 11, 2001, terror attacks. His advisers acknowledge that he is far less known to voters than the leading Democratic candidates. His debut television commercial stressed his credentials as a self-made executive — “a middle-class kid who made good” — and his political advocacy on core Democratic concerns like gun control and climate change, as well as on economic development and public health issues like smoking. Those issues, his advisers say, will all be central to his candidacy.

While Mr. Bloomberg is a political moderate, his opening message also borrowed in some respects from the anti-Washington rhetoric of the Democratic Party’s populist wing, led by progressives like Mr. Sanders and Senator Elizabeth Warren of Massachusetts. “I know how to take on the powerful special interests that corrupt Washington,” Mr. Bloomberg said in his statement on Sunday. “And I know how to win.”

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Gov. Gina Raimondo of Rhode Island, who helms the Democratic Governors Association, said she had spoken recently with Mr. Bloomberg and saw him as a strong candidate despite the late hour. Ms. Raimondo said the former mayor recognized he faced “long odds” in the primary, but she suggested Mr. Bloomberg could be well positioned in a general election to take on Mr. Trump’s perceived credentials as a businessman.