In the Saddleback debate, McCain confessed that his "greatest moral failure" was with respect to his previous marriage. When McCain returned from Vietnam, he discovered that his wife, a former swimsuit model, had been in a severe auto accident, leaving her physically disabled, four inches shorter, and substantially heavier than when he last saw her. McCain did not keep his marriage oath: "for better or for worse, in sickness and in health, to love, honour, and cherish, 'til death do us part."
What if McCain takes the oath of office as President to "preserve, protect and defend the Constitution of the United States," and then something more attractive than the Constitution walks his way?
Then there's the matter of what happened to McCain in Vietnam. An organization called "Vietnam Veterans Against John McCain" has been raising some serious questions since at least July of 2005, when their website went online with CSPAN videos of McCain's conduct in Congress.
Sydney H. Schanberg has done some research for the Nation Magazine:
Why Has John McCain Blocked Info on MIAs?
and a longer statement of his research funded by the Nation Institute:
McCain and the POW Cover-up
The facts seem to suggest that we cannot trust John McCain to keep the promises that really matter (marriage, constitution) but we probably can trust him to keep his campaign promises to bail out Wall Street fascists and empower Washington D.C. socialists.