If Sen. Susan Collins runs for a fifth term, she ought to expect a very different race than in the past. Forget coasting to victory, no matter the opponent or even the nature of the election cycle.

Collins will start off as vulnerable — a top Democratic target in a state carried by Hillary Clinton in 2016.

The Maine Republican’s great strength over the years has been her moderation and thoughtfulness. She mulls over issues extensively, almost always looking for middle ground.

She supports abortion rights and LGBT issues, and she has broken with her party on topics ranging from the environment to taxes to the Affordable Care Act.

But Collins’ vote to confirm Brett M. Kavanaugh to the Supreme Court gives Democrats an opportunity to retire her next year — as does the fact that her contest could determine which party controls the Senate.