With hundreds, even thousands, of bills to navigate over a career, there is danger in every ambiguous vote, like the one pitting insurgent hordes against caribou herds.

As political handicapper Charlie Cook put it, "A career in the Senate is like a paper trail of little time bombs ready to blow up."

Indeed, in the history of the republic, just two senators have gone directly from the Senate to the White House. John F. Kennedy was one. The other was Warren G. Harding.