You're in charge of any campaign you want (post 1980), and you can change one thing. What is it?

[RINO Tom]

Romney 2012, of course, as his failure to win the election likely had the worst recent consequences, IMO, especially if we consider Trump to be one of those (which I do).  I would have picked a more moderate running mate with a more "blue collar" Midwest appeal (like Kasich?) rather than using Ryan as a "look how conservative I am!" move.  Conservatives were already very energized to vote Obama out, and a GOP primary electorate with very high participation willingly nominated Romney.  I feel he let the echoes of his primary rivals have too much influence.  I also think he really thought (as did many of us) that we would have a reversion back to an electorate a bit more like 2004 with the shine of Obama's 2008 candidacy worn off and didn't adopt a strategy that looked forward.

People can point to specific policy things, but that didn't make a damn difference in that election.  JUST enough swing voters who had no problem voting for John Kasich as governor (OH) in 2010 or Dean Heller for Senator (NV) found Obama to be relatable enough to vote for.