Who’s done the bigger 180 on this guy — Dems or Reps?

Well that depends on what time frame your meaning wouldn't it?

Mitt has/had been a known Democrat even before the time when someone thought he would make a good controlled opposition to the then CIA asset in the White House.
You should have been aware if your a former republican voter that Mitt Romney has not been liked for a long time and probably would not have his current position without Trumps endorsement.

So If you mean in the last week then Democrats have whiplash and broken necks. No question in the last week who.
However, if your implying it was Republicans then just put it in their perspective, Mitt is was and always will be 2 faced Trash as is anyone who continues to oppose or go against the general view of any party they are in will be considered. He is McCain 2.0 in what he has done without quite the traitorous past.
His actions were only to stick it Trump,Romney has no more integrity than a Lamp Pole he is a politician after all.

Sure, there were always folks within the GOP that didn’t like “moderate” candidates like McCain and Romney. As shown by the fact that McCain and Romney didn’t crush their primary opponents right away. They had significant opposition from candidates who were stronger fiscal or social conservatives

But by general election time, Republicans all rallied around the candidate their primary process chose. Didn’t they? And if they instead stayed home on Election Day: does it make sense to think of them as solid Republicans in the first place?

Whatever individual conservatives’ personal feelings, it’s a fact that Mitt Romney was the GOP’s standard-bearer in 2012. And all real conservatives would have been happier if Romney won than Obama.

Ass kicking time...so much fun~
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In 2008 and 2012 the GOP put up some really shitty candidates for the office. They had their butts handed to them by a candidate that was only slightly less of a shitstorm. In 2016 the roles were reversed due to the libs lowering the bar even further and having clinton on the ticket. 2020 is going to yield the same results. The slate of candidates out there for the Dems all have one thing in common. They want to turn this country back towards the cliff of ultimate doom