Registered Republicans did not show up at the polls. Romney got the independent vote that voted for Obama in 2008.
The Republican-leaning independents mentioned below, are they conservative purists or squishy moderates?
...In the end, the enthusiasm edge may have helped drive right-wing true believers to the polls in disproportionate numbers, and self-described conservatives outnumbered acknowledged liberals by 35 percent to 25 percent, according to exit polls. But this advantage couldn’t compensate for the no-show status of Republican-leaning independents...