That presidents often say one thing and do something seemingly inconsistent is hard to dispute.

However, typically those inconsistencies, to be unmemorable, tend to be on something other than the candidate's "core platform."

When a candidate runs on, basically, one "core platform" (be it the economy, family values, the environment or whatever), and it is shown that they are hypocritical in that one area, it can spell death for the campaign.

For example, Bush Sr ran on "read my lips, no new taxes." Later raising taxes, more than anything else, killed his campaign in 92. His flip flop meant there was nothing left for him to define himself with.

McCain's whole "schtick" is "ethics in government." That's fine, but once its shown he's exempting his contributors from that, it will make him look very bad. He'll have nothing left to define or differentiate himself from, for example, Hillary Clinton.