The first primary debate of the season was held on May 5th in South Carolina. Only five candidates chose to participate: Cain, Paul, Pawlenty, Santorum, and former governor of New Mexico Gary Johnson. Political commentators suggest that Pawlenty was the only "top tier" candidate who attended. In fairness to Paul, however, his loyal constituency is usually enough to get him the poll numbers needed for a debate invitation (until the requirements are tightened later in the season).
The other candidates--Cain, Santorum, and Johnson--will need to boost their poll numbers to ensure they get a chance to participate in the debates to come. Johnson's numbers are so poor that he has not even been included in the Elephant Watcher roster of candidates. As for the others, you may see a more detailed analysis of their strengths, weaknesses, and strategies on their Profiles.
So who won the debate? In a political debate, of course, there is no such thing as "winning the argument." Instead, candidates aim to achieve certain goals. In this case--even for Pawlenty--the goal was to attract media attention. The debates also present an opportunity for the candidates to prove their rhetorical ability. So who succeeded?
No candidate performed in an outstanding way. Two candidates, Santorum and Johnson, performed poorly. But Cain, Paul, and Pawlenty each achieved what they set out to do:
Cain--who has never held elective office--managed to look like a respectable candidate who deserved to share the stage with the professional politicians. Most likely he did not generate quite as much excitement as he would have hoped, however.
Paul performed for his fans, and he gave them exactly what they wanted. Paul seeks to keep his constituency intact and enthusiastic. Thus far he has not shown any inclination toward expanding his base.
Pawlenty did not make any major stumbles, did not pick fights, and performed ably. He is often criticized as a boring candidate, but he avoided being too stiff. He was the most polished of the candidates. Arguably he did the best job.
As for the candidates who did not perform as well: Santorum's only strength is his social conservatism, which he was not able to use to articulate a reason why he ought to be president. Johnson had a somewhat embarrassing debate. At one point, he complained about not getting enough questions (though actually each candidate was given a fair share), which calls to mind the abortive candidacy of Mike Gravel in 2008 (who was referenced in an earlier post about debates).
In summary, the debate produced little of any particular note. It was the first debate, and it showed: It seemed like something of a warm-up exercise. If a candidate seeks to propel himself into the lead with a good debate performance, he will need to show something more.