In a surprise announcement, Mayor Michael R. Bloomberg said Thursday that Hurricane Sandy had reshaped his thinking about the presidential campaign and that as a result, he was endorsing President Obama.
Mr. Bloomberg, a political independent in his third term leading New York City, has been sharply critical of Mr. Obama, a Democrat, and Mitt Romney, the president’s Republican rival, saying that both men had failed to candidly confront the problems afflicting the nation. But he said he had decided over the past several days that Mr. Obama was the better candidate to tackle the global climate change that he believes might have contributed to the violent storm, which took the lives of at least 38 New Yorkers and caused billions of dollars in damage.
“The devastation that Hurricane Sandy brought to New York City and much of the Northeast — in lost lives, lost homes and lost business — brought the stakes of next Tuesday’s presidential election into sharp relief,” Mr. Bloomberg wrote in an editorial for Bloomberg View.
“Our climate is changing,” he wrote. “And while the increase in extreme weather we have experienced in New York City and around the world may or may not be the result of it, the risk that it may be — given the devastation it is wreaking — should be enough to compel all elected leaders to take immediate action.”
Mr. Bloomberg’s endorsement is another indication that Hurricane Sandy has influenced the presidential campaign. The storm and the destruction it left in its wake have dominated news coverage, transfixing the nation and prompting the candidates to halt their campaigning briefly.
The announcement is also the latest in a series of steps Mr. Bloomberg has taken in a bid to assert his influence nationally as his final term as mayor enters its twilight — and after he appears to have abandoned his own hopes of one day becoming president.
Last month, the mayor said that he was creating his own “super PAC” to support candidates from either party, as well as independents, who he believes are devoted to his brand of nonideological problem solving, and he has increasingly used his personal wealth and the bully pulpit of his office in an effort to persuade elected officials to support same-sex marriage, gun control and education reform.
The impact of Mr. Bloomberg’s endorsement is unclear; his city and his state are overwhelmingly Democratic, and although he is a well-known and long-serving public official who frequently appears in the national media, his influence is difficult to measure: an ABC News/Washington Post poll conducted in December found 30 percent of Americans had a favorable view of Mr. Bloomberg, 26 percent had an unfavorable view, and many — 44 percent — had no opinion of him one way or the other.
Both the Obama and Romney campaigns had aggressively sought the mayor’s endorsement, in large part because they believed he could influence independent voters around the country. Mr. Bloomberg had recently signaled he would not make an endorsement, telling reporters several weeks ago that he had decided whom he would vote for, but that he was not sure he would share that decision with the public.
John Weaver, a prominent Republican political strategist, said the timing of the mayor’s endorsement was notable.
“His announcement is sandwiched between this horrific calamity and the presidential election,” he noted. “So the timing could not have been more significant for him and his views.”
Steve McMahon, a veteran Democratic strategist, said he believed that now that Mr. Bloomberg had come to terms with not running for the presidency, he was interested in cementing his political legacy.
“Many politicians reach the point in their careers where they have built up considerable political equity and the only question is how they use it to make a difference,” he said. “In endorsing President Obama, the mayor seems to have decided to use some of his equity.”
Even before the hurricane struck, Mr. Bloomberg had been concerned about climate change. He is the chairman of an organization called C40, a network of cities seeking to reduce greenhouse-gas emissions.
Yet until the storm, climate change had not been much of an issue in the presidential campaign. The topic did not come up during the three presidential debates, and the candidates have not provided detailed legislative or regulatory plans outlining their stances on the issue.
Since the hurricane, a number of elected officials have come forward, chiefly in New York, to say that they have concluded the planet is undergoing climate change, that huge storms are no longer freak occurrences but expectable reality, and that public policy must begin to prepare for the impact.
Gov. Andrew M. Cuomo of New York said Tuesday, “Anyone who says there is not a dramatic change in weather patterns I think is denying reality.” Senator Charles E. Schumer, Democrat of New York, said Wednesday, “We’re going to pay a price for the change in climate.” And Representative Jerrold Nadler, Democrat of Manhattan, echoed the consensus of local officials on Thursday, saying simply, “There will be a storm of this magnitude again.”