To John McCain's credit, and Sarah Palin's chagrin, John McCain did correct people in the audiences of his speeches and town halls who wrongly believed that the President was a Muslim. To someone like John McCain, where integrity, honor and truth seem to matter a great deal, I suppose not doing so, and allowing such malicious disinformation that is fostered on many corners of the right to grow, was a rubicon he could not cross.
Rick Santorum has no such trouble.