My quick rating of the four Republicans still standing:
Romney - He's the frontrunner. The same qualities that worry conservatives are likely to attract moderate swing voters. He also has the best-organized campaign and is the only candidate left with actual executive experience. That gives him the best chance of beating President Obama. It is not a sure thing but it is a shot. He still says things that can be used against him. He is naturally friendly and open, more than people are used to which makes him seem a little strange.
Deep down conservatives know that Romney is no Reagan and he will break our hearts but he is still an improvement over Obama.
Gingrich - His campaign is fading fast. He finished Michigan behind Ron Paul. And conservatives with long memories remember that he already broke our hearts. He became made numerous promises when he became Speaker of the House. He was going to eliminate whole departments and de-fund PBS. He didn't accomplish many of his promises. Instead he led his party to higher-than expected losses in 1998 which led to his resignation.
Paul - He is the first to admit that he is not in the race to win. He just wants to keep the Libertarian wing from being forgotten.
Santorum - If Romney will break conservatives' hearts, Santorum will break the party. He appeals mainly to the most hard-core social conservatives and has no appeal to moderates or libertarians. Santorum could pull down the entire ticket and deliver the House back to the Democrats.
So, the best choice is to hold your nose and vote for Romney - advice that Clinton gave the Democrats in 1996 about his own candidacy.