Flake can't swing it, don't think Romney could either.
I’m not so sure.
One of two things will happen in 2020
1. Trump will get annihilated due to his historic unpopularity and increased Democratic engagement/turnout
2. The election will be very very close (the last one came down to 80,000 votes split across 3 states).
Scenario 1, a traditional conservative independent run wouldn’t make any difference, obviously. Scenario 2, it would only take a fairly small number of votes siphoned off to make Trump’s chances all but disappear.