(Image: Jon Geeting)

New York City could become the largest U.S. city to adopt ranked-choice voting this November, if voters there pass a ballot referendum recommended by their Charter Revision Commission. A win there could help provide a momentum boost to recent efforts to bring RCV to Philadelphia and Pennsylvania. 

Politico reports that the measure has attracted broad support from groups across the political spectrum, in part because there's a lot of disagreement about what it would mean in practice for different parties and factions, though everyone seems to think it'll be good for their own interests. 

Ranked Choice Voting, and specifically Instant-Runoff Voting in this case, would allow voters to rank their preferences instead of choosing only one candidate, as voters do under the "First-Past-the-Post" approach that's in use in most places today. 

The way the instant run-off works is that after the voting ends, there are several elimination rounds where the candidates with the least support are eliminated from the pool, and their votes are reallocated to people's second choices, and on and on until the candidate(s) with majority support prevail. Because voters' ranked preferences are already known, this is all tallied up instantly and there's no need to have an entire separate run-off election on a different day, as New York City does now. 

Currently, when no candidate for Mayor, Public Advocate or Comptroller wins 40% or more of the vote in a citywide primary election, that sets up a second run-off election between the top two vote-getters. One of the talking points from ranked-choice voting supporters in New York is to note that these run-off elections see extremely low turnout, and are much less representative of the city than the larger primary electorate. If we can just take down people's ranked preferences the first time, there's no need for everybody to have to come out and vote a second time a few weeks later. 

How ranked-choice voting impacts politics overall will vary according to the politics of the place in question, and the other election rules and institutions in place, but two of the key things it does are eliminating the "spoiler" effect in primaries, and creating incentives for candidates to try and win lots of second and third-choice votes. 

A common move you see in big-city politics is that when an incumbent politician gets a challenger, the incumbent will recruit a second challenger to try and split the anti-incumbent vote. With ranked-choice voting, this strategy becomes much less effective. In a hypothetical three-way race with an incumbent, a challenger, and an incumbent-recruited spoiler, if the spoiler has the least support of the three, the spoiler is eliminated first, and her votes are reallocated to her voters' second choice. If the incumbent doesn't get an outright majority and win the election in the first run-off, the original challenger is able to consolidate the anti-incumbent vote in the end. 

The need to win lots of second and third-choice votes, in addition to first-choice votes, also creates a strong incentive for candidates to run positive campaigns rather than go negative on other candidates and anger their supporters. In places that use ranked-choice voting, like San Francisco, you'll sometimes even see ads for candidates endorsing one another and asking voters to give their second-choice vote to candidates running on similar platforms, to increase the odds that one of them will win. 

If the NYC ballot initiative wins next month, it could give a nice boost to recent efforts like Sen. Anthony Williams's new bill, which would establish ranked-choice municipal elections for the city of Philadelphia. 

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