A part of New England, Maine was once a bastion of the Republican Party, and was one of two states to vote against Franklin Roosevelt all four times he ran for president.

With the GOP progressively becoming more culturally conservative, the state began moving toward the Democrats, and Bill Clinton carried it in 1992.

[8] On January 12, 2015, Maine Governor Paul LePage made a statement on Howie Carr's radio program that he might run for U.S. Senate against King, citing King's switching his endorsement in the 2014 gubernatorial election from independent candidate Eliot Cutler to Democratic candidate Mike Michaud as a "horrible thing to do".

[10] The next day, LePage said his comment was a joke,[9] though on an August 25, 2015 appearance on Carr's program, he said he was "very strongly" considering running, citing King's caucusing with Senate Democrats.

[12] On May 10, 2016, LePage announced at a town hall meeting in Oakland that he would run against King unless he was hired by the Donald Trump administration.

[21] The Democratic primary race initially included both teacher Zak Ringelstein and homebuilder Benjamin Pollard, but Pollard withdrew to run as an independent two days after Ringelstein announced a list of 16 endorsements from Democratic state legislators.

[35] with Paul LePage Counties that flipped from Independent to Republican King won both congressional districts, which both elected Democrats.