McCain is staunchly opposed by a vocal group of conservatives who view him as an unreliable maverick. You can hear their most prominent advocate on the radio every weekday from noon to three eastern. You can see them in the exit polls, which show that McCain has not yet won a (statistically significant) plurality of Republican voters, nor those who consider themselves 'very conservative.' In years past, opposition to the Republican frontrunner tends to fade away after South Carolina, with the supporters of the loser accepting that their guy can't prevail and reconciling themselves with the victor. But that does not seem to be happening this year. There is a faction of the party that seems unwilling to accept McCain. It might be able to stop him. It should be clear from the nomination rules that somebody could find enough delegates to oppose McCain on the convention floor - even if he did not offer a serious challenge early in the process.
Right now, it seems that the anti-McCain faction is more anti-McCain than pro-Romney or pro-Giuliani. But that could change.