Here is the Romney campaign's revisionist history: 

Campaign surrogates repeatedly pointed to Santorum’s 18-percentage point loss in his 2006 Senate re-election campaign, arguing that voters had punished Santorum for abandoning conservative ideology. "The reason he got beat, I think, was that he moved so far way from his fiscal conservative principles," Rep. Billy Long (R-MO) argued on the call.

Byron York clarifies

The biggest policy reason for Santorum's loss was his outspoken support for the war in Iraq.  

… "As other Republicans attempt to steer away from Iraq and terrorism, Sen. Rick Santorum argued yesterday that America must stop 'sleepwalking' while 'evil enemies' plot the nation's destruction, making foreign policy a focal point in the final days of his campaign," the Philadelphia Inquirer reported on October 27, 2006. 

Santorum lost independent voters by 43 points. Larison revisits the numbers: 

Going back through the CNN exit poll for the Pennsylvania Senate race, I found a few more numbers that cast doubt on Santorum’s competitiveness in a general election. He lost moderates by 30 points (65-35), Catholics by 18 points (59-41), suburban voters by 14, and college educated voters by 22 (61-39). Even granting that he was the incumbent in a very anti-Republican election, these are not the numbers of a general election winner.