Full disclosure: I voted GOP top to bottom in 2012, including Romney/Ryan.
But let's admit two things. First, if Romney had won in 2012, 2014 would have been a Democrat reaction, not a GOP takeover--and while the Congress is still pro-Obama, the state houses are where the fight is happening now. Second, If Romney were running for re-election, Hillary!/Castro would use Trump or someone else as 3rd party a la 1992 to beat him in '16, when the more likely scenario now is Trump and/or Cruz will beat Hillary!/Castro, and we will be GOPe-less as well as Dem-less.
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There is no cogent scenario where Trump beats Hitlery.
(Hitlery owns no hotels)
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Hardly a given.
while the Congress is still pro-Obama
Delusional.
Romney would have nipped ISIS in the bud rather than letting it grow to crisis proportions, there would be no Iran deal, the Keystone pipeline would have been approved, we'd probably be figuring out how to dismantle Obamacare right now, Obama's lawless immigration stuff wouldn't have happened, all this BLM crap wouldn't be happening, plus a thousand other things.
The second order benefits of losing don't offset the first order benefits of winning, even with an imperfect candidate.