For one thing, Mike Huckabee won more delegates in 2008 than Romney, so he had as good a claim to the "next in line" title. But he decided not to run. Had Mitch Daniels or Haley Barbour gotten into the race, either could have raised serious money. Similarly, if Jon Huntsman had declined to serve in the Obama administration, he would have been much more credible to Republican primary voters and he would have had more opportunity to build his organization and warchest. By the time he actually ran, however, most Republicans knew little about him, and the rest thought he was tainted by the Obama connection.
Any of these three could have taken a big chunk of the traditional/establishment vote that went to Romney, thus making him more vulnerable to a credible challenge from the right.
That challenge could have come from Rick Perry -- if had started earlier and worked harder. But Byron York nailed it back in October:
For Romney, debate preparation involves taking all the things he has already thought through and finding the most effective way to present them in one-minute answers. For Perry, debate preparation is trying to learn new stuff about national issues that he should have been thinking about a long time ago.
It's often pointed out that since Perry entered the Republican race late, on Aug. 13, he had little time to build a campaign organization and hone a campaign pitch. That's true, but the fact is, if Perry wanted to be president, he should have been thinking seriously about the substance of national issues -- not just money-raising and state chairmen -- years before he declared his candidacy.Even before his famous "oops" moment, his campaign was heading south.
Bachmann, Cain, and Gingrich had moments in the sun, but none of them ever had a real chance of nomination. Santorum, on the other hand, might have made life tougher for Romney if just a couple of things had gone differently. If the Iowa GOP had produced an accurate vote count on caucus night, the story would have been "Santorum edges Romney in stunning upset." If Santorum hadn't disrespected JFK, he might have carried the Catholic vote in the Michigan primary, which could have been enough for him to score yet another stunning upset.
In other words, "the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all." -- which is the Bible's way of saying "You didn't build that."