Rep. Karen Bass has built a double-digit lead in the Los Angeles mayor’s race with a little over two months to go until election day, firming up her base among the city’s Democratic voters and eroding Rick Caruso’s margin in the San Fernando Valley, a new poll shows.

Since beating Caruso in the June primary by 7 points, Bass has widened her advantage over the businessman to 43%-31%, with 24% undecided, according to a new UC Berkeley Institute of Governmental studies poll, co-sponsored by the Los Angeles Times.

Bass has consolidated support among liberal and Democratic voters, picking up the lion’s share of those who went for other candidates in the primary. She leads Caruso by nearly 2 to 1 among former supporters of Councilmember Kevin de León, who came in third in the primary, and wins overwhelmingly among backers of Gina Viola, the progressive activist who came in fourth.

Bass has also significantly narrowed the lead Caruso enjoyed in the Valley, the one region of the city the businessman won in the primary. The Valley accounted for 38% of ballots cast in the primary, and he won there by 7.5 percentage points. Now he’s up by just 2 points, according to the poll.

These numbers raise the question of whether the billionaire mall developer, who bombarded Los Angeles’ airwaves with millions of dollars of advertising during the primary, can claw back into contention and make the race more competitive as the runoff between the two moves into its final phase.

“This isn’t a done deal” because of Caruso’s vast resources, said Paul Mitchell, a political data expert who has been closely following the race.

“It’s a 12-point lead where you’re going to have a large amount of spending, and Caruso is going to have an opportunity to try to re-introduce himself to voters and also to try to be more effective than she is at turning out the voters that do support him.”

Nonetheless, Caruso starts out the fall campaign in a deep hole. Although determining who is a likely voter is difficult this far in advance of the election, among voters whose responses to the poll indicate that they are most likely to cast ballots, Bass’ lead grows to 21 points — 53%-32%, with 14% undecided.

Bass, a six-term member of Congress representing parts of the Westside and South Los Angeles, has several advantages going for her — some of which her campaign has generated, others which involve changes in the overall political environment.

Those advantages start with a far more favorable image among voters than Caruso has.

About half of registered voters surveyed, 49%, said they had a favorable opinion of Bass, while 22% said they had an unfavorable view, and 29% gave no opinion.

Caruso’s numbers are starkly worse. Thirty-five percent of respondents had a favorable impression of him, while 40% said they had an unfavorable view and 24% had no opinion.

“The favorable-unfavorable comparison between the two candidates is really striking,” said Mark DiCamillo, who directed the poll and has been surveying California voters for decades.

Bass has built her popularity and her sizable lead on support among registered Democrats, people who identify as strongly liberal and Black voters as well as liberal white voters. These groups make up the majority of voters in Los Angeles.

She leads Caruso by 40 points among registered Democrats, 30 among voters who describe themselves as somewhat liberal and nearly 70 points among those who say they’re strongly liberal.

Caruso has found some success with moderates, a sizable chunk of the electorate among whom he has a nine-point lead. Among people who identify as strongly conservative, who make up a small chunk of the city’s registered voters, he has a 50-point advantage.

The candidates’ images are similarly polarized along partisan and ideological lines, the poll found.

“Her image among Democrats is overwhelmingly positive, and his is overwhelmingly negative,” DiCamillo said. “It almost looks like a Democrat versus a Republican on the ballot even though that’s not technically the case.”

The poll found that Bass and Caruso were essentially tied among Latino and Asian voters, with about 34% of the vote. Black voters favored Bass by more than 40 points and she was up among whites by 17 points.

In L.A. precincts with populations that are at least 80% Latino, Caruso got 34% of the vote in the primary, and Bass 27%, according to a Times analysis. Turnout in these Latino-heavy precincts analyzed by The Times was just 17% — much lower than the overall turnout of 30%.

This is the second poll in as many weeks to show Bass up big. A poll released by an outside group supporting the congresswoman found her up by 11 percentage points among likely voters.

When asked what attributes they wanted in their next mayor, voters in the Berkeley IGS poll leaned toward traits that more closely mirror Bass’ background than Caruso’s.

The poll found that 71% of voters said it was important to have someone who is progressive, 75% want a mayor with previous experience in elected office, and 72% want someone who has a history of defending abortion rights.

The one attribute that might favor Caruso is that 77% want a candidate who is tough on crime.

Voters were far less interested in whether the candidate had a business background, was a political outsider or was a woman.

“If you look at those top four that were all 70% or above, only one of those would probably not be associated with Bass as opposed to Caruso,” said USC political science professor Ange-Marie Hancock Alfaro — with crime being the exception.

In February, when The Times asked similar questions, respondents rated previous elected experience and progressive politics as the two most important attributes of a future mayor. In this poll, people who said previous experience was very important supported Bass by about a 40-point margin. Bass had similar advantages among people who said it was very important to have a progressive mayor and those who wanted a mayor with a long history of defending abortion rights.

Among voters who said it was very important to have a candidate who is tough on crime, Caruso leads 47% to 29%.

He has made crime a major focus of his enormous advertising blitz in the spring — along with homelessness. But Bass so far appears to have been able to defang the issue.

In July, Caruso vociferously attacked Bass for endorsing city attorney candidate Faisal Gill, who has run partly on a promise to institute a 100-day moratorium on the prosecution of most new misdemeanor charges. The congresswoman then revoked her endorsement of Gill.

Earlier in the campaign, she also endorsed more hiring at the Los Angeles Police Department.

While those stands angered some activists on the left, they do not appear to have cost Bass many votes among progressives, even as they have blunted Caruso’s attacks.

Caruso’s ads “were effective in kind of defining him during the primary season and getting him to where he got, but I think that was more of a ceiling than I expected,” Hancock Alfaro said.

Bass has also benefited from “what’s changed on the national front,” Hancock said — the renewed focus on women’s reproductive rights in the aftermath of the U.S. Supreme Court’s decision overturning Roe vs. Wade.

Bass’ campaign has pointed repeatedly to her long support for abortion rights and Caruso’s position on the issue. A journalist in a 2007 Los Angeles Magazine profile paraphrased Caruso’s views on the subject, writing: “He says he opposes abortion in most cases but would support some stem cell research.”

Caruso’s campaign has declined to comment on those remarks, but throughout the primary the candidate said he would be a stout defender of reproductive rights if he were elected. He also has castigated the Supreme Court’s June decision.

Bill Carrick, who previously advised mayoral candidate Jessica Lall and is now working with Robert Luna in his campaign for Los Angeles County sheriff, said Caruso’s previous Republican affiliation is “the elephant in the room.”

Caruso needs to do a better job of explaining why he’s a Democrat, Carrick said, and sharpen his explanation for why he switched parties.

Caruso switched between Republican and no party preference multiple times before becoming a Democrat in January.

Still, Carrick, a longtime expert in California politics, said Caruso will be an exuberant campaigner this fall who will spend big to get his message across.

“He’s clearly planning an aggressive field operation and has obviously stepped up his retail campaigning,” Carrick said.

Bass may also benefit from the recent failure on the part of opponents of Dist. Atty. George Gascón to get a recall on the ballot.

If the signature drive had succeeded, it might have been on the November ballot, although that was not a certainty. If it had been, it probably would have increased turnout among conservative voters who disapprove of Gascón and favor Caruso.

Supporters of the recall favored Caruso 57% to 24%, the poll found. Caruso, a former LAPD commissioner, earlier this year said he supported the recall, while Bass opposed it.

Had the recall gotten on the ballot, it would have been a serious threat to Gascón. The poll found that registered voters countywide would have favored the recall 41% to 20%, with the rest undecided. Citywide the margin was slightly tighter, with 37% favoring the recall, 23% opposed and the rest undecided.

The UC Berkeley Institute of Governmental Studies poll was conducted Aug. 9 to 15, among 4,538 registered voters in Los Angeles County. It included 1,746 voters in the city of Los Angeles, of whom 1,212 were deemed likely voters. The estimated margin of error for the registered voter sample is +/- 2.5 percentage points for the countywide sample and +/- 3 percentage points for the city.

Times staff writer Iris Lee contributed to this report.