If the Democrats are serious it will be Dean or another governor. Congress critters are too tainted.
Say WHAT!?!?!? I stopped reading at that point.
If you're speaking from the Congress straight to the Presidency, you're right. But what is even more rare is for a Representative (as opposed to a Senator) to go from the House to the White House. I think it was only done once: by Republican James Garfield. He was assassinated about six months into his term, I believe by a bureaucrat who wanted to suck more off the government's teat.