Only way to stop it is for all Democrats to flip to republicans and surge vote for his primary challenger. May it be Jeff Flake. 100% Dems plus 25% republicans can win him the seat. A registered republican can still vote Dem in the general, so no need to worry about being stuck having to vote for a republican in the general.
Dems are to weak to win in 2020. I haven't heard all the reports of Reps. in the rust belt flipping back to Dem so you have to assume they will vote for Trump again. We heard about it in '16 when they left the Democratic Party and those are the only states that matter in '20. I expect the rest of the states to stay the same.