So what is acceptable as a reason to change one's mind? More information. Additional time to reflect more deeply about a problem. Changed conditions.
Most of all, Americans consider a flip-flop acceptable if they trust a leader to do the right thing. That was the case with Franklin Roosevelt. After promising to balance the budget in the 1932 campaign, he took office and began an unprecedented run of deficit spending. But Americans thought his expensive social programs helped them to weather the Depression, and they forgave FDR one of the biggest flip-flops in U.S. history. Moral: Reversals don't necessarily doom a leader—as long as the new policy seems to work better than the old one.
Corrected 08/08/08: An earlier version of this article incorrectly referred to the tax issue. It should have said that McCain now supports the big tax cuts enacted during Bush's first term.