Maine will soon become the first state to adopt ranked-choice voting in presidential elections.

Gov. Janet Mills, a Democrat, announced on Friday that she would allow a bill recently passed by the Maine Legislature to become law without her signature. The first vote conducted under the new law will be the general election in November 2020.

Under the new system, voters will be able to rank as many candidates as they like in order of preference. The initial count will look only at their first choices, and if one candidate receives a majority, that candidate would win.

If no one receives a majority, however, the candidate with the fewest first-choice votes will be eliminated, and his or her votes will be redistributed to those voters’ second-choice candidates. This process will repeat until one candidate breaks 50 percent.

Proponents say the system ensures that the candidate with the broadest appeal wins. Effectively, it prevents third-party candidates from becoming “spoilers” by siphoning a decisive number of votes from one of the major-party contenders, resulting in a winner that a majority of voters oppose.