On November 6, 2012, King won the Senate race with 53% of the vote, beating Democrat Cynthia Dill and Republican Charlie Summers. The following week, King announced that he would caucus with Senate Democrats, explaining not only that it made more sense to affiliate with the party that had a clear majority, but that he would have been largely excluded from the committee process had he not caucused with a party. King said he had not ruled out caucusing with the Republicans if they took control of the Senate in 2014 United States Senate elections, but when Republicans did win the majority that year, he remained in the Democratic caucus. King remained in the Democratic caucus after the 2016 and 2018 elections, both of which also resulted in Republican Senate majorities.