Start with Rick Santorum, whose poll numbers in New Hampshire and South Carolina have been surging since (by last count) he lost the Iowa caucuses by the Chinese lucky number of 8 votes.
Santorum's weakness is that he can't resist concentrating on peripheral issues. The prime example is his leadership in 2005 in getting the Senate summoned into voting for a law preventing the removal of life support for Terri Schiavo.
Santorum's position was intellectually defensible (and shared by Democrats like Tom Harkin). But voters considered it weird to devote so much energy to a single unhappy legal case. I think this accounted more than anything else for Santorum's 59 to 41 percent defeat in Pennsylvania in 2006.
In New Hampshire Santorum was unable Thursday to resist Boston radio talk host Michael Graham's invitation to characterize himself as a "Jesus guy." Again he had an intellectually coherent rationale.
And at an appearance in Windham that evening he made it clear that he doesn't believe a candidate has to be a Christian -- a necessary concession in a nation whose Constitution bars any religious test for office.
But in discussing these issues, he needlessly gave credence to those who dismiss him as nothing more than a religious conservative when in fact he has a serious record on, and has been talking about, economic and foreign issues.
Jon Huntsman, even more dependent on a breakout in New Hampshire than Santorum, has a different weakness. His disdainful dismissal of other Republicans, even more than his service as Barack Obama's ambassador to China, has antagonized many conservatives.
At the same time it has attracted people who, like Huntsman supporters I've interviewed after his town halls, characterize him as the least bad alternative among the Republican candidates. And it's netted him the endorsement of liberal papers like the Concord Monitor and the Boston Globe.
Yet Huntsman's positions on economic issues are solidly conservative and have been praised by the Wall Street Journal. On education he is a thoughtful backer of school choice. As governor of Utah he took many smart conservative initiatives.
The tension between the anti-conservative aura he gives off and his genuinely conservative positions seems to have left Huntsman between two stools and struggling to achieve the solid third place finish in New Hampshire that might plausibly give him a ticket to other states.
Other candidates' weaknesses are so obvious that they can be quickly summarized. Ron Paul looks and sounds zany.
Yes, his attacks on the Federal Reserve are more plausible than they were four years ago, and he's on his way to doubling his percentage in New Hampshire as he did in Iowa. But he's not going to attract more votes with a brochure cluttered with arcane verbiage and with keywords in ALL CAPS.
As for Rick Perry, all you have to do is watch that agonizing 53 second brain freeze again. Perry's weakness is that he's never had a secret ambition to be president of the United States.
He got into the race when other politicians' decisions not to run seemed to create an opening for the governor of the nation's second largest and best job-creating state. But his sketchy knowledge of national and foreign issues revealed him as a man who had already achieved his life's ambition as governor of Texas.
Newt Gingrich's weaknesses? Where do we start? On the stump in Iowa he was constantly detouring off message, and his pledge not to campaign negatively was at odds with the attack dog tactics that enabled him to end Democrats' 40-year majority in the House.
But his greatest weakness, I think, is that he sees himself as a world historical figure. Being derailed in Iowa by negative ads pointing up the $1.6 million he pocketed from Freddie Mac seems to him as enraging as if Winston Churchill had been shoved aside in May 1940 because of his (genuinely) dodgy finances. But rage doesn't attract votes.
Which leaves Mitt Romney, well ahead in New Hampshire polls, ahead in post-Christmas polls in South Carolina, with the resources and organization needed to win in giant Florida.
His weakness is that he never experienced the cultural revolution of the 1960s and so sounds corny and insincere. So far that hasn't been disabling.
Michael Barone,The Examiner's senior political analyst, can be contacted at email@example.com. His column appears Wednesday and Sunday, and his stories and blog posts appear on washingtonexaminer.com.