Rep. Bass is backed by several high-profile Democrats for L.A. mayor, with one notable exception.

As the race for Los Angeles mayor has tightened to a dead heat between Representative Karen Bass and billionaire real estate developer Rick Caruso, the veteran congresswoman has deployed a who’s who of endorsements from fellow Democrats.

President Biden. Hillary Clinton. Senators Alex Padilla and Bernie Sanders. Antonio Villaraigosa, the former mayor, was beside her for her campaign kickoff. Former President Barack Obama endorsed her recently.

Increasingly striking, however, has been one notable absence: Gov. Gavin Newsom, who has steadfastly refused to endorse either candidate, both Democrats.

“I have deep respect for both of them, and I have not gotten involved in that race,” Mr. Newsom said in a television interview this summer after a runoff narrowed the field in the officially nonpartisan race to the two contenders.

Mr. Caruso, he noted, had been “incredibly helpful and supportive” on a statewide economic development task force the governor had created during the pandemic, and Ms. Bass was “a remarkable leader going back — well before my time, even — in politics.”

In other statements, the governor has added that he prefers not to endorse in contests between two fellow Democratic contenders. Mr. Caruso, a former Republican who has spent nearly $100 million dollars, most of it his own money, on a record-setting campaign for the city’s top office, registered as a Democrat shortly before he announced his candidacy. A spokesman for the governor responded to a request for comment with his former remarks.

But political observers in Sacramento and Los Angeles note that the governor has abandoned that stance in at least two races. In a hotly contested state Senate contest in Sacramento, Mr. Newsom has endorsed a Sacramento City Councilwoman, Angelique Ashby, over Dave Jones, who won the state party’s endorsement. And in a race for Los Angeles County supervisor on the affluent Westside of the city, he endorsed the former state Senate Majority Leader Bob Hertzberg over the mayor of West Hollywood, Lindsey Horvath.

Mr. Caruso and Mr. Newsom also share a political consultant. But other clients of that firm, Bearstar Strategies, including Sen. Padilla, have endorsed Ms. Bass.

“I think this is personal for him,” Bill Carrick, a longtime Democratic strategist in Los Angeles, said on Thursday. “I think he feels like he has a good relationship with both of them, and from everything I know, that’s true.”

And, Mr. Carrick and others said, there are obvious downsides for the governor in picking a favorite.

“Karen Bass has been a good, strong Democrat for a long time and is a supernice person, and if you’re the governor of California, you don’t want to antagonize the entire Democratic base,” said David Townsend, a veteran Democratic political consultant in Sacramento. “But if you’re thinking about your future, you also don’t want to pick fights with people who have a lot of money.”

Mr. Newsom’s national profile has risen sharply in the year since he beat back a Republican-led recall, and his name has come up as a future contender for the White House, although he has insisted that he has “sub-zero” interest in a presidential run.

In liberal Los Angeles, it was unclear how much difference Mr. Newsom’s backing would make at this point, with the voting deadline only days away.

“It’s interesting to insiders,” said John Shallman, a Los Angeles political strategist with long experience in local races. “But I don’t think voters would care — and for her, it wouldn’t matter unless it came with resources.”

Mr. Caruso has outspent Ms. Bass by more than 10-to-one, he noted: “She needs more than names — she need money now.”