The problem was largely with the fact that Obama lost eight million voters and Romney failed to pick up any signficant number of them. He actually came in with a lower total of voters than McCain in 2008 - and McCain was functioning in an epic bad year for Republicans whereas Romney had a Carteresque opponent - and thought just running against Obama would suffice. But he offered little to the voters leaving Obama, so they simply did not vote.
The recipe for success was demonstrated in 2010. Grassroots, vigorous conservatism that drew in the dissastified Obama swing voters by offering a robust alternative to both progressives and wampy RINO moderation. And the response of the GOP-E? Treat the Tea Party as unwanted relations. And that led directly to their loss. The Tea Party in the end even tried to help push that tried-and-failed jalopy over the finish line - but it was too late.
The trick is not to be more like Democrats. The GOP loses when it tries that. The trick is to state clear conservative values - and MEAN it. Romney never meant a word of it, so his words had no attractive value to the crossover voters of 2010.
I do have a slight disagreement in that there were a lot of purist conservatives and evangelicals that would have met the McCain standard of votes (thereby winning), but stayed home. I was pleased when JR and most of FR changed and became ABO voters. There was a purist faction that stayed the anti-Romney course, sizeable, who sat out, voted third party, or deliberately didn't vote because Romney was a Mormon. I gather from the numbers there were enough of them to throw Bambi out.
I blame them as much as some of the mistakes Romney made. I still don't believe anyone else who ran (or Palin who did not run)could have done better or come closer to winning. They all were as deeply flawed as Romney in different ways, without some of his strengths.