Susan Collins was never vulnerable to a 2020 primary challenge for 2 reasons:
1. Republicans are smart enough to know that it would cost them the seat. If they were not this smart, she would have been primaried in 2014.
2. The Democratic wave of 2018 was guaranteed to knock out all the potential high profile primary challengers, meaning that Collins would never have been up against a serious candidate in a primary.
1. The Trump wing is not smart, it cares about loyalty. They will happily nominate an extremist if it means more loyalty to the president. Plenty of examples of this happening in 2018.
2. Her most prominent possible primary challanger is LePage, who cares little about a democratic wave in 2018.
