Forget the popular vote. It's meaningless (as Al Gore proved).
Forget the insane things Santorum says that turn off voters.
Forget the stupid things Romney says that turn off voters.
There are only a dozen or so states that are truly up for grabs. Pennsylvania is one of them.
The second-biggest swing state next to Florida went to Obama in 2008 in large part because of the strong campaigning that Scranton-native Joe Biden did. If Mitt Romney is the nominee, Pennsylvania is safely in Obama's corner. If Rick Santorum is the nominee, it will be a fight and Santorum has a good chance of winning it.
Florida could go to the GOP if they have Marco Rubio as the VP.
The scenario below paints a closer general election than most are willing to admit. Santorum's popularity in Colorado, Ohio, and Indiana make him a more-formidable challenger to Obama than Romney.
In short, Romney mathematically cannot win without a major flub by Obama or other circumstances (economy, gas, Iran, etc.) that could paint his Presidency poorly in the short-attention-span voting populace of America.
If Santorum can deliver Pennsylvania and a few other states, he is the bigger threat to Obama.
That's how I see it. Bash away at the theory!