Why did Mitt Romney select Paul Ryan as his running mate

[Pennsylvania Deplorable]

After Walker's victory in Wisconsin, I think Romney thought he could win WI with Ryan's help. Ryan was also known as a budget hawk and the national debt was still a big GOP talking point back then. Ryan also added a more youthful look to the campaign. Of course, just being younger didn't automatically make him a compelling figure who could connect with people and he fell flat on every occasion. I think it was down to him or Rubio. Rubio might have helped Romney win Florida, but he wouldn't have helped much elsewhere.

[Pennsylvania Deplorable]

I don't know why Ryan wouldn't be popular with the base ... the fact that some now call him of all people a "RINO" is truly telling of the Breitbart crowd.

I think it started with him surrendering on the debt ceiling issue and passing omnibus (he was before that adored by hardcore fiscal conservatives). His attacks on Trump even during the general election cycle, support for mass immigration, and (real or perceived) support for establishment republican incumbents over tea party challengers all contributed as well. Trump also instilled in the base a deep suspicion of GOP megadonors like the Koch brothers and (not without reason) Ryan is seen as their puppet.