Follow along with our coverage of the Democratic National Convention.

Michael R. Bloomberg, who bypassed his own run for the presidency this election cycle, will endorse Hillary Clinton in a prime-time address at the Democratic National Convention and make the case for Mrs. Clinton as the best choice for moderate voters in 2016, an adviser to Mr. Bloomberg said.

The news is an unexpected move from Mr. Bloomberg, who has not been a member of the Democratic Party since 2000; was elected the mayor of New York City as a Republican; and later became an independent.

But it reflects Mr. Bloomberg’s increasing dismay about the rise of Donald J. Trump and a determination to see that the Republican nominee is defeated.

Mrs. Clinton is seeking to reach out to middle-of-the-road swing voters and even moderate Republicans uneasy about Mr. Trump. Polls show that significant numbers of Republicans remain wary of Mr. Trump, and question his fitness for the presidency.