There is a more subtle point to be made here. Hypocrisy is frequently expressed as the divergence of one’s words with one’s actions. I’d alter that traditional definition a bit. I would say that hypocrisy is the divergence of one’s words from one’s true beliefs.
Let me give two examples. Joe is a public campaigner against overeating. Joe is 400 lbs. Joe may not be a hypocrite. He may merely be weak, a much less damning offense. He knows what a trap food has become for him and he wants to help others avoid his fate.
John is a public campaigner against overeating. John’s primary business is selling deep fried Twinkies at parties. John is more likely to be a hypocrite. We can more easily believe that he really doesn’t have a problem with people who have bad eating habits but that he merely says those things publicly because he thinks they will make him more popular.
I must admit that I haven’t read much about Ensign’s affair and the aftermath. Nor do I know anything about him as a person. So instead, let me speak to a hypothetical politician in his same situation. We’ll call him Fred.
Fred actually believes the things that he spoke publicly about morality and marriage. He recognized the weakness in himself that tempted him to violate his marriage vows. He recognized the damage breaking those vows would do to his family and to society at large. Thus, he took public positions that sought to encourage others to uphold those vows and virtues. Eventually, however, Fred succumbed to temptation and did what he knew he shouldn’t do. He still believes that what he did was wrong and is willing to publicly admit the same.
So far, Fred is not a hypocrite. He may be unfit for public office, but that is a separate question of trustworthiness and judgment.
Now suppose that Fred had frequently called for other politicians who strayed to step aside and relinquish their posts. But when his own failing comes, he decides not to step aside. Now I would say that Fred is a hypocrite. (I’m looking at you, Mr. Sanford.) He believes that what is right for him is different that what he’s been saying all along.