All republicans have to do is call the opposing candidate a radical socialist and they win Florida.
As for no-longer-battleground states, I would say Ohio, Iowa, New Mexico, Colorado, and Virginia.
NM wasn't really a battleground in the 2012 and 2016 elections, and considering that the Democrats' winning margin in the Senate race was relatively poor (the GOP didn't really target it, the consensus was that NM Sen is somewhere between Likely and Safe D), I would be cautious to write it off completely. Definitely more likely to flip than CO or VA.
