San Francisco mayoral candidates Jane Kim and Mark Leno intensified their “anyone but London Breed” campaign Thursday, promoting an ad paid for by both campaigns that urges voters to list them as their first two picks on the June 5 ranked-choice ballot.

It’s an unusual tactic that hasn’t been used to this extent in the 14 years San Francisco has had ranked-choice voting. However, Jean Quan was part of a similar tag-team effort in Oakland nearly eight years ago, and it helped propel her past a better-financed front-runner into the mayor’s job.

“I’m proud to be the first set of candidates ... to truly take advantage of the ranked-choice voting system and encourage our supporters to vote for both of us,” Kim said at a news conference with Leno on the City Hall steps.

Their 30-second ad features Kim and Leno standing side by side as they warn that “wealthy special interests are trying to buy this election.” Although the ad doesn’t mention Breed by name, the Board of Supervisors president has been the top recipient during the campaign of spending by independent expenditure committees, commonly called Super PACs.

Kim, a two-term supervisor, and Leno, a former state legislator and supervisor, endorsed each other as their preferred second choices weeks ago. Several polls show that the race is a tight one, and if their voters go along with the strategy, it could put one of the two in the mayor’s office — even if Breed comes out with more first-choice votes.

In 2010, former state Sen. Don Perata was well ahead in first-choice votes in the Oakland mayoral election but didn’t win a majority in the first round. Quan, a city councilwoman who had adopted an anyone-but-Perata strategy with fellow candidate Councilwoman Rebecca Kaplan, eventually prevailed.

Under the ranked-choice system, voters can list first, second and third preferences. If no candidate receives more than 50 percent, the last-place candidate is eliminated and his or her votes are redistributed. The process is repeated until one candidate reaches a majority.

In 2010, Quan told her supporters to make Kaplan their second choice, and Kaplan told her supporters to make Quan their No. 2. When the time came to redistribute Kaplan’s votes, Quan won 75 percent of them.

Teaming up in an election with a ranked-choice voting system, Quan said Thursday, “is one of the best chances that minorities and progressives have to coalesce and garner the votes” against a better-funded opponent.

“If Jane and Mark can convince people they’re the progressive candidates compared to London, it could help them if they can get people to vote them as their one and two,” Quan said.

Breed’s campaign dismissed the tactic.

“What’s the news here?” said Breed’s campaign spokeswoman, Tara Moriarty. “Leno and Kim’s one-two strategy was announced to the world the day they orchestrated London Breed’s removal as acting mayor.”

The reference was to the Board of Supervisors’ vote in January to select Mark Farrell as mayor, after Breed had filled the post in an acting role after the death in December of Mayor Ed Lee.

John Whitehurst, a political consultant who helped run Perata’s 2010 campaign, said a key distinction between that race and San Francisco’s special election is that Perata was a divisive figure in Oakland politics. His negative reputation with some voters made him vulnerable to a ranked-choice tag team, Whitehurst said.

“Whereas London, Jane and Leno are all very popular with voters, and very few voters dislike them,” said Whitehurst, who worked with Leno during legislative campaigns but is not advising any mayoral candidate. “Perata had earned a generation of battle scars. He had a huge base that was deeply loyal and supportive, but there was also this huge base over the years that didn’t like him. There’s not that same negative image of London.”

On Thursday, Kim and Leno pointed to an attack ad funded by Gayle Conway, wife of wealthy tech investor Ron Conway, who has worked to raise money for Breed’s campaign.

The ad goes after Kim for joining three other progressives on the Board of Supervisors in 2012 to reinstate Ross Mirkarimi as sheriff, after he was convicted of misdemeanor false imprisonment for an incident in which he bruised his wife’s arm during an argument.

Leno said the attack ad was “a distraction” from the city’s homelessness, housing affordability and public-safety problems.

The Leno and Kim campaigns split production costs for their tag-team ad. So far it is running only online, but the candidates said it will eventually be aired on television.

Outside groups have spent just over $500,000 backing Breed, according to data from the San Francisco Ethics Commission. Kim has benefited from $230,000 from such groups, and $55,000 has been spent to support Leno.

In addition, the Gayle Conway-funded committee behind the anti-Kim ads has spent nearly $180,000, commission records show.

By law, such committees can spend unlimited amounts on a campaign but cannot coordinate their activities with a candidate.

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa