The odd alliance between these mayoral rivals ahead of the June 5 election is a function of San Francisco’s system of ranked-choice voting, whereby residents can choose up to three candidates by order of preference instead of the usual one. Kim and Leno each want voters to put themselves as their first choice, but they’re urging their supporters to select each other as their second choice on Election Day. Ranked-choice voting has been in place for municipal elections in San Francisco for 15 years, but this year’s race is the most wide-open campaign for City Hall during that time and the first in which two major candidates have so explicitly taken advantage of the system by forming an alliance.

Advocates of ranked-choice voting cheered the move as a validation of an election reform they hope to expand to other major cities, such as New York, and to states as well. Maine next month will become the first state to use ranked-choice voting in its primary elections for governor and other offices.

But the Kim-Leno alliance is not without controversy. By endorsing each other, they are effectively trying to thwart the candidacy of the putative frontrunner in the race, London Breed, the Board of Supervisors president who served briefly as the city’s acting mayor following the December death of Mayor Ed Lee. In a move that enraged Breed supporters, her colleagues on the board—including Kim—voted to name another member, Mark Farrell, as interim mayor through the election. The editorial board of the San Francisco Chronicle, which endorsed Breed, accused Kim and Leno of “gaming” the ranked-choice system and wrote that the move “projects an element of desperation.”

Each of the three leading candidates would mark a first for San Francisco if they win. Breed would become the first African American woman elected to lead the city; Kim would be the first Asian American woman to serve as mayor; and Leno would be the first openly gay mayor in the city’s history. In heavily Democratic San Francisco, all three hopefuls are staunch progressives on the issues that animate national politics: They back same-sex marriage, a higher minimum wage and government spending, and all of them oppose President Trump’s crackdown on immigration and support San Francisco’s status as a “sanctuary city.”

But on local issues, Breed is seen as the most moderate of the top three candidates in the race, which has been dominated by a debate over the city’s twin crises of affordability and homelessness. In press interviews, Kim and Leno have knocked Breed for siding more frequently with developers, and they’ve criticized her association with a tech investor who endorsed her candidacy at a memorial service for Lee and then helped fund super PACs backing her bid on the airwaves.

“When it comes to asking the private sector to do more, or to follow along with rules, or to have rules even that benefit greater society as a whole, that’s where London and I have been apart,” Kim told me. “We always say: Everyone is progressive until money is involved in San Francisco.”