The number of crossover votes for Collins is going to collapse in 2020 compared to the last time she ran in a presidential year. In 2008 probably about 20% of all Maine voters cast ballots for Collins and Obama. I would be really surprised if that number is more than 8-9% in 2020.
For the moment Collins in favored but it will be close. I think Maine is one of the few states that could swing slightly to Trump even if he loses, just given the demographic shifts we saw in 2016/18. Plus the Democrats will throw a decent amount of money on this race, which may go far since it's a small state.
If I had to guess now, I'd guess that Collins wins by 2-3% while the Democratic nominee wins by 3-4%
Collins is doomed
No she is not. Her race is Lean Republican, and if Trump manages to come within single digits again, like in 2016 (and I believe he will), she will probably win, or at least come close. My current prediction is that she hangs on by about 3-4 percentage points, like Manchin and Tester did last year.
Poll
