Why did Mitt Romney select Paul Ryan as his running mate

[Southern Senator North Carolina Yankee]

At the time of Ryan's selection (late summer), Romney was still very much on the track of trying to win the Midwest to win the election.  It wasn't until September/October that he saw the writing on the wall in Ohio and Michigan and Wisconsin and started shifting more resources back to "traditional" Republican states like FL, VA and CO.

Paul Ryan was a young, conservative legislator from the Midwest who came from a humble background (unlike Romney). 

FL, VA and CO were not enough to win the election without Ohio.

[Southern Senator North Carolina Yankee]

Not memeing, but Chris Christie would've been Romney's best bet. He was hugely popular in 2012.

The Romney vetting people uncovered dirt on Christie that made him a bad choice.

[Southern Senator North Carolina Yankee]

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.

How would a think tank shill who has not had an original thought that wasn't fed to him by people still stuck in 1986 (more think tank shills from an older generation), be popular with the base of the Republican Party?