PORTLAND – The city’s Charter Commission supports a new system known as ranked-choice voting, or instant runoff voting, to ensure that Portland’s mayor is elected only with a majority of the popular vote.

There’s just one problem: the math.

An election with ranked-choice voting could still produce a winner who’s not supported by a clear majority of the voters. That’s what happened in a mayoral election in Burlington, Vt., a year ago and in several Board of Aldermen elections in San Francisco.

To address that issue, commission members are considering whether to allow for a second, runoff vote between the top two finishers in the first election.

To ensure the legitimacy of a popularly elected mayor, it’s important to require support from the majority of people who vote in an election, said Commissioner Nathan Smith.

On occasion, he said, that may require a second runoff election.

Critics say the commission should just abandon the whole idea of ranked-choice voting and adopt a more conventional system with two elections — the first to narrow the field to two finalists and the second to choose a winner.

That would be less confusing to voters, said Terry Reilly, a former election official in San Jose, Calif., who has become a vocal opponent of ranked-choice voting.

“As cool and hip as instant runoff voting sounds,” Reilly said, “Portland has the benefit of learning from other cities and do like the rest of the (nation’s) 20,000-plus municipalities and not experiment with a new voting scheme that has proven to fail on so many fronts.”

The commission, which has a July 1 deadline to recommend changes to the city government, is expected to resolve the issue at its meeting Thursday. Portland voters will vote on proposed charter changes on Nov. 2.

The primary proposal calls for a popularly elected mayor with a four-year term. Currently, the City Council selects one of its members to be mayor for a one-year term.

Ranked-choice voting is essentially a series of runoff elections, tallied in rounds. Here’s how it works:

On ballots, voters rank candidates in order of preference by filling in the first-choice bubble next to their favorite candidate, the second-choice bubble next to their second favorite, and so on.

After the polls close, the first-choice votes are counted for all candidates. If no candidate gets a majority, the ballots are recounted and the last-place candidate is eliminated.

The remaining candidates get any second-choice votes cast for them on the ballots won by the eliminated candidate.

In later rounds, if a second-choice candidate has already been eliminated, voters’ third-choice votes are redistributed.

The process repeats until one candidate has a clear majority.

That would produce a majority, if not for human nature. In practice, some people vote only for their favorite candidate, and nobody else.

In a mayoral election in Burlington that used ranked-choice voting, the winner of a six-person race was elected after three rounds, with a margin of 48 percent to 45 percent. What happened to the missing seven percent of the vote? More than 600 voters didn’t rank either of the two finalists on their ballots.

In San Francisco, which has used ranked-choice voting since 2004, all 10 members of the Board of Supervisors who were elected using ranked-choice voting received less than a clear majority of votes, according to research that Reilly provided to The Portland Press Herald.

A subcommittee of the charter commission met Monday to examine Reilly’s arguments. Rob Richie, executive director for FairVote, a Maryland-based group that advocates for election reform, attended the meeting via teleconference. Richie had successfully lobbied the commission on the issue last year.

Richie acknowledged that it’s possible for a candidate to win without a majority under the ranked-choice system. But when traditional runoff elections are used to decide a clear winner, he said, far fewer people typically vote in the second election than in the first.

Ranked-choice voting, he said, is the best method for selecting a candidate with the broadest support.

And in an election with multiple candidates, he said, the method ensures that the winning candidate is not someone opposed by a majority of voters.

Smith, the member of Portland’s commission, said there is no ideal solution, “but we are working in a universe of imperfect ways of doing things.”

Staff Writer Tom Bell can be contacted at 791-6369 or at: tbell@pressherald.com