Obama got a record number of votes that year but not at all an unusual amount compared to Bush four years earlier.
What it looks like is McCain and Romney both failed to get normal growth ~ and instead not only lost normal growth, they lost some of the core Republican voters.
That would be, of course, the Rockefeller wing ~ Romney's own peeps! They just didn't show up and they didn't bother telling people they weren't going to do that, and nobody in those states went out and sought to find more Republicans in a registration drive!
I really can't comment on national popular vote totals unless you can guarantee me that they were real and reasonably accurate. I have a hunch that due to various types of fraud, including fraud in the counting of votes by electronic devices, Romney's reported national popular vote may have really been higher than officially reported. One can hack into a vote counting device to cause an undercount for the candidate whom you want to lose, just as easy as you can cause an overcount for the candidate whom you want to win. Also possibly contributing to a falsely low reported vote total are instances when absentee ballots for Republican candidates are arbitrarily discarded by compromised election board workers or instances when local election boards failed to get ballots out to absentee voters in a timely fashion. There were reports of such instances of the latter type which effectively disenfanchised military personnel abroad.
IIRC, the total popular vote reported for all presidential candidates was less in 2012 than in 2008, without any apparent logical reason for such a dropoff.