Top 12, eh? Hmm...
AZ is the most obvious flashpoint as long as Sinema is running as independent
WV, MT, OH all red to deep red states with Democratic incumbents
NV is a swing state with small senatorial margins
TX just because
Gosh, I'm only halfway? WI PA IN FL VA MN
That's for the general election, mind you. There are other states where the primaries will be interesting; or rather, I expect there will be, but I don't have a firm grip on which ones. I suppose California would be pretty high on the list, actually.
Actually, shit. California does the jungle primaries. So the general might be a D vs. D showdown against Feinstein if she doesn't retire first.