If it did happen, then yes, it's a crime. That's not really up for debate. It's law.
Sestak kind of fucked himself there. True or not, he won't benefit either way. And he didn't admit this to "out" the Obama admin., he did it to promote himself as the underdog "outsider." So he can't even ride the "Washington whistle-blower" angle.
But I have a feeling it'll blow over before it gets big. It's a boring story and nobody'll care.