Commissioner Nathan Smith, who chaired a subcommittee that researched the issue, said he has become convinced that the system is better than a primary or a runoff election because there is typically a sharp drop-off in voter turnout in a second election.
Also, holding a second election would be more costly for the city and the candidates, he said.
In a single plurality election with multiple candidates, a well-organized minority faction could elect a mayor with only narrow public support. Under ranked-choice voting, that would not happen, he said.
Portland is the largest city in Maine, making it a potential model for adopting instant runoff voting elsewhere in the state. Currently, the mayor of Portland is not an elected position, but if the ballot measure reforming the city's charter passes, the office of mayor would become a citywide elected position, selected by instant runoff voting.
Portland is not the only major city where voters may soon decide if they want to adopt instant runoff voting. New York City's Charter Revision Commission is also looking closely at adopting it. From “TheWall Street Journal” (behind paywall):
The proposal, known as instant runoff voting, would allow voters to rank candidates in order of preference. If a candidate fails to reach 40%, the threshold in New York City for winning a party's nomination in citywide races, the ballots would be counted again, with voters' rankings used to simulate a runoff. [...]
The Charter Revision Commission plans to release Friday a report that explores the possibility of instant runoff voting. The 15-member commission intends to solicit public input on the concept and decide by early September whether to place the proposal on the Nov. 2 ballot.
New York currently has traditional top-two runoff elections, which tend to see large dropoffs in voter participation for the second round of voting. If the city adopts instant runoff voting, it would be the largest political entity in the United States to use it. Currently, a few Americans cities like Minneapolis use the system, which the United Kingdom will possibly adopt for its House of Commons election. Australia uses it extensively at all levels of politics, include the national House of Representatives.
If New York City implements instant runoff voting, it would be a large step forward for those hoping to end the zero-sum politics produced by our first-past-the-post election system. A significant portion of New York State residents live in the city, giving instant runoffs real potential to spread to statewide elections. New York's tradition of fusion ballots and important minor parties makes it potentially more receptive to this kind of reform.