My numbers come from the U.S. Census Bureau and the Atlas of U.S. Presidential Elections.
In 2012, the total voting age population (U.S. Citizens over the age of 18) was 215,081,000. The total number of registered voters was 153,157,000.
Obama got a total of 65,918,507 votes, which equals approximately 30.6% of total voting age population or 43% of all registered voters.
Romney got a total of 60,934,407 votes, which equals approximately 28.33% of total voting age population or 39.8% of all registered voters.
That leaves approximately 40% of the voting age population or 17% of the registered voters who stayed home.
Now, imagine if the Republicans had nominated a strong conservative candidate for President in 2010. You don't think a strong conservative candidate could have could have gotten some of those missing 40% of the voting population and 17% of the registered voters to the polls?
By way of comparison, Bush got re-elected in 2004 with 31.5% of the total voting age population or 43.7% of the registered voters. Bush was a moderate/liberal. The only reason he got re-elected was enough conservatives were scared enough of how close we had come to having President Algore on 9/11 that they came to the polls to vote against President Kerry. Certainly, Bush did not win re-election because of his "compassionate conservative" tax and spend domestic policies.
If Mittens Romney had only done as well as Bush (either as a percentage of the total or registered vote) he would have won.