Mitt Romney attracted fewer voters than did John McCain in 2008, or W. in 2004. Similarly, Barack Obama attracted fewer voters than he did in 2008. In other words, many potential voters stayed home.
It was a negative campaign. Team Obama said Romney was a wealthy corporate raider who didn't want to bail out GM and Chrysler and opposed abortion. Their negative ads were mostly correct.
Team Romney said Obama was a failure as a president, did essentially nothing to reverse unemployment, and, with the exception of killing Osama ben Laden, most of his 'accomplishments' were disliked by the public. Team Romney's criticisms were mostly correct.
You have to wonder how many potential voters found the negative ads of both campaigns convincing and were not motivated to vote for either? Apparently the numbers were quite large.
I saw one analysis which put the number of potential Romney voters who stayed home at 10 million. If a third of these had voted we'd be talking about President-elect Romney.
The GOP needs to discover who stayed home, why, and what it can do differently to get these people to vote GOP.