Susan Collins announced Friday that she will not run for the governor's seat in 2018.
The 64-year-old Collins made the announcement Friday morning at an event in Rockport.
Ms. Collins, a Republican who was first elected to the Senate in 1996, has become a thorn in the side of President Trump, for whom she did not vote.
Collins read aloud a letter from a Senate colleague: "The institution would suffer in your absence".
"As I thought about the senator's words, I realized how much needs to be done in a divided, troubled Washington, if we are to serve the people that we represent effectively".
"I am a congenital optimist, and I continue to believe that Congress can - and will - be more productive", Collins added. She is not up for re-election in the Senate until 2020. As one of the few moderates in a closely divided Senate, she is often a swing vote, and, as she demonstrated during the health care debate, she can often influence the outcome of important legislation.
Her announcement ends widespread speculation that the 20-year veteran of the Senate would return to ME and seek to finish her political career in the governor's mansion - a position denied to her in 1994 when she finished third in a five-way race. She also said she felt an affinity for Augusta, the capital, where a long line of ancestors served in the State Legislature, starting with her great-great grandfather and including her father.
"This decision has not been any easy one".
Three Republican candidates are now in the race.
And that's that the Maine Republican Party still has faces the choice between being the party of Susan Collins and Bill Cohen or the party of Paul LePage and Donald Trump.
Republicans are now trying for a sweeping rewrite of the nation's tax laws, which many of them hope will wash away some of the deep anger in the party after the health-care failure.
She would join a crowded field in the race for governor to replace two-term Republican Gov. Paul LePage, who can not run again because of term limits.