Here's John McCain's campaign referencing the CNN's exit polls:
Exit polls found 64 percent of Tuesday's Republican voters still support the conflict — and Romney, whose criticism of Bush's management of the war has been muted, led McCain among those voters. But among the 34 percent who said they disapproved of the war, McCain had a wide advantage over the GOP field — even over Texas Rep. Ron Paul, the sole advocate of a U.S. withdrawal in the Republican field.
John McCain brings credibility to the issue of the war—so much that he, the most ardent advocate of the war—attracts the votes of those who oppose it. That means something in the general election. People who want to see the war through and bring along others who oppose it in order to unite America have a champion in John McCain.
So Republicans who disapprove of the war flocked to John McCain, and not to Ron Paul? How strange!
Update: After giving this statistic some thought, I have a possible explanation for it. I've always thought that Americans -- and Republicans in particular -- do not despise the Iraq war in principle. If the war were going better, they'd be behind it. This means that given the choice between a "resolute" anti-war candidate, like Ron Paul, and someone they thought could actually win the war, they would opt for the latter.
This is why I've been suspicious of polls that show that "80% of Americans are against the war in Iraq," etc. Such polls never distinguish between those who would reject the war even if it were going well and those who reject the war only because they think the United States is losing.
My guess is the first group is much smaller than the second, and that the first is mostly comprised of hard-core leftists, pacifists, and other assorted hippies. Since anti-war absolutists are almost always in the minority, it's unthinkable that the large number of Americans who once supported the war have suddenly all joined this group of people.
Americans will support war, as long as they have confidence they will win.