I think almost anyone could have run a better campaign than Mitt did in the general election, including Rick, and that should have been obvious from how he did in the primaries. He won by massively outspending his opponents, and whenever he didn't, he was in trouble.
That said, it's difficult to see how Rick could have won, though I think at the very least he would have kept Florida in the GOP column and had a decent shot at picking up Ohio and Virginia. But that only gets one up to 266 and he's not really a great match for any of the next tier of states. Maybe Iowa, but he have had to really work at it to have even a small shot, and avoided getting diverted into the socon trap.
Rick would have had little chance of picking up Ohio, and no chance whatsoever ay Virginia. The NOVA numbers would have been off the charts brutal with Santorum as the nominee.
As far as Santorum being the sole reason, of course that is an absurd remark. The 47% remark comment obviously hurt as well. However, there is no question the Primaries pulled Romney to the right and it hurt him with the center in November.
