San Francisco Mayor London Breed endorsed Mike Bloomberg for the Democratic presidential nomination, boosting the billionaire’s chances in a delegate-rich state where he got off to a late start but is now spending millions to try to win.

Breed, who backed California Sen. Kamala Harris until she withdrew from the race in December, told The Chronicle exclusively Wednesday that she “thought long and hard” before deciding that Bloomberg’s “track record of what he’s done as New York City mayor and what he’s done afterward has been significant.”

Breed added, “He has the ability to beat Donald Trump this November, and that is of the most concern to me.”

Endorsing Bloomberg, who made his $54 billion fortune off his eponymous media empire, carries political risk in San Francisco, where rising wealth inequality has reshaped the city. Breed also will undoubtedly hear from the city’s progressive community about Bloomberg’s long support for the stop-and-frisk policing policy during his 12 years as mayor.

As for the other Democrats in the race, Breed said, “If there was someone else who could (beat Trump), I would be supporting them.” She added that she, like Bloomberg, would support whomever the Democratic nominee is.

Breed said she will advise the Bloomberg campaign about jobs, homelessness and housing, and climate change. She said she has reached out to Bloomberg for advice on homeless issues, and he has shared ideas on how best to acquire land for low-income housing.

When it comes to endorsing a billionaire in a campaign where progressive candidates Sens. Elizabeth Warren and Bernie Sanders have higher taxes on the wealthy at the center of their platforms, Breed makes no apologies.

“Instead of being selfish” and just thinking about New York, Breed said, Bloomberg has tried to use his fortune to “improve the climate and the quality of life in other communities.”

Breed joins San Jose Mayor Sam Liccardo and Stockton Mayor Michael Tubbs as former Harris backers who are now behind Bloomberg. Los Angeles Mayor Eric Garcetti, Sacramento Mayor Darrell Steinberg and Long Beach Mayor Robert Garcia, another former Harris backer, have endorsed former Vice President Joe Biden. Oakland Mayor Libby Schaaf is not endorsing a candidate in the primary.

“Voters re-elected London Breed by a wide margin because she is taking on the biggest and toughest issues and she puts progress over politics,” Bloomberg said in a statement. “I’m honored to have her support and look forward to working with her not only to win this election, but to help make San Francisco and all of California stronger, fairer and greener — with more affordable housing, more good jobs, and health care for all.”

Bloomberg, who has swamped his Democratic rivals with $200 million in TV advertising since he entered the race in November, has pulled into a virtual fourth-place tie nationally with former South Bend, Ind., Mayor Pete Buttigieg, according to the Real Clear Politics average of major polls. Biden, Sanders and Warren are ahead of them.

Doing well in California is key to Bloomberg’s unorthodox strategy of skipping early-voting states such as Iowa and New Hampshire. He is focusing first on the 14 states including California that hold contests March 3.

Breed said she was impressed by Bloomberg’s championing of climate change issues and how the high school graduation rates for African American students increased in New York while he was mayor. She said his Greenwood Initiative plan, which would invest $70 billion into low-income communities with the goal of increasing African American home and business ownership, is “better than any plan I’ve seen” to help the community.

Yet Breed said that “of course” she had concerns about Bloomberg’s support as mayor for stop-and-frisk police policies, under which New York officers detained people they thought might be engaged in crime and searched them for weapons or other contraband. Bloomberg apologized at a Brooklyn church last month for his past support of stop-and-frisk, under which the vast majority of those detained were African Americans and Latinos.

Breed said she and Bloomberg “had conversations about it. ... It was a failed policy. He made it clear that it was a mistake.”

Tubbs, the Stockton mayor who endorsed Breed, told The Chronicle last month that for people like him, a 29-year-old African American man, “stop and frisk is terrible.” He recalled having “a spirited discussion” with Bloomberg, but that he was satisfied with the candidate’s apology.

“That’s a sign of a good leader,” Tubbs said. “I’m sure the mayor will put forth policies that will reflect his new criminal justice perspective.”

Former Philadelphia Mayor Michael Nutter, who is African American and serves as a top surrogate for the candidate, said Bloomberg slowed the pace of the program toward the end of his term. New York recorded 649 homicides in 2001, the year before Bloomberg became mayor, and 335 in 2013, his last year.

Nutter said he didn’t think it was cynical of Bloomberg to apologize for the program at a predominantly black church just days before launching his campaign.

“I think it would be cynical to think of it as anything other than a heartfelt apology,” Nutter said last week while accompanying Bloomberg on a campaign stop in Oakland. “You will rarely see, anywhere at least in America, that a current or former official, having realized that they made a mistake ... (going) directly to the people who are most affected by that damaging aspect, standing up in public and saying, ‘I’m sorry.’”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli