That Kavanaugh speech basically sealed her fate in 2020. Especially with how the Maine election played out.
Ironbite-might as well resign now so the GOP can try and wedge someone else in there.
As I've seen noted, Collins (like Manchin) has shown a remarkable ability to defy gravity. The last time she faced an unfavourable environment, after all, was 2008, where Obama won Maine by more than 17 points over McCain, but Collins won by almost 23 points--a 40-point swing. (Under more favourable conditions in 2014, she won by 37, while Obama won by a little over 15 points two years prior.)
She might also have been gambling on a closer Senate (51-49 or 50-50) than actually resulted, which would have given her a lot more influence than she has in a 53-47 Senate. As it is, she (and Lisa Murkowski) are effectively marginalized, as are other GOP Senators who have shown flashes of principle on various issues. (Think John Kennedy on net neutrality, or Rand Paul and Mike Lee on certain foreign policy matters.)
I honestly wouldn't be surprised to see her make an independent run in 2020, figuring that she can't be re-elected as a Republican (even if she can win the primary) and that she couldn't win a Democratic primary if she jumped parties entirely. At least then she'd have a chance of picking up enough second-preference votes in a three-way race (assuming Maine's system is ruled constitutional, since Rep. Poliquin is challenging it in court after he lost on second preferences despite leading on first preferences).