Santorum did quite well head to head against Romney until he dropped out. During contested primaries it was quite clear that the majority of Republican primary voters did not want Romney as the nominee. However, Romney managed to help slime much better conservatives than he right out of the race.
Others have posted it, and I will repeat it: people were warned how horrible a candidate Mitt Romney would be and were warned more than once.
And I will also answer the question before it's asked: the beauty of Ronald Reagan was that he could bypass a media that was as unfriendly to him as it has ever been to any Republican to take a message directly to the American people. Did we have a candidate who could do that who was not named Romney? We will never know, because Romney's GOP-E helped take them all out of the race.
Obviously we didn't.