The full text of his announcement can be found on the new City Room blog.

Mr. Bloomberg announced his decision after a campaign-style swing through California in which he gave a series of speeches that clearly previewed what aides have long said would be the thematic underpinnings of a Bloomberg presidential campaign, should he decide to run.

Image Mayor Michael R. Bloomberg talked to reporters on Tuesday in Los Angeles. Credit... Reed Saxon/Associated Press

He presented himself as an antidote to partisan gridlock in Washington, suggesting that not withstanding his affiliation with the Republican Party, he brought nonpartisan government to New York.

“When you go to Washington these days, you can feel a sense of fear in the air, the fear to do anything or say anything that might affect the polls or give the other side the advantage or offend a special interest group,” Mr. Bloomberg said. “The federal government isn’t out front — it’s cowering in the back of the room.”

Should Mr. Bloomberg ultimately decide not to run for president or any other office, his announcement could become an interesting footnote to one of the more unusual mayoralties in a city that has produced a series of memorable mayors.

But Mr. Bloomberg’s decision to leave the Republican Party and become an independent was immediately viewed by many of his prospective rivals as a major jolt to the presidential campaign. Mr. Bloomberg has never shown any reluctance to use his huge personal fortune to advance his career. He spent more than $150 million on his two bids for mayor, and he would have no problem financing his own presidential campaign.

What is more, Mr. Bloomberg has arguably at least as strong a claim on New York City’s prosperity as his predecessor, Rudolph W. Giuliani, who is seeking the Republican nomination. If Mr. Bloomberg decides to run as an independent or third-party candidate, he might find that he enjoys the benefits of New York City’s successes without the ideological burdens that Mr. Giuliani has faced in trying to win the Republican nomination while being identified with such positions as supporting abortion, gay rights and gun control.

That said, several political analysts have argued that a third-party candidacy by Mr. Bloomberg could pose a problem for the Democratic Party. Until he ran for mayor, Mr. Bloomberg was a lifetime Democrat, and his success in New York reflected his ability to draw Democratic votes.