Possibly the best analogy for Hillary Clinton's continuing campaign can be found at MotherJones.
Essentially, she's the dead parrot in the famous Monty Python skit. As long as she says she's in the race, she's in the race. Since there are no experts able (or willing) to say otherwise. So on the campaign goes, and John McCain continues to run without opposition, and without scrutiny.
(You could also say she's the man from "Monty Python and the Holy Grail" who insists "I'm not dead! I feel fine! I feel happy!" and then gets put out of his misery by a club-weilding John Cleese. But Obama supporters can only wish.)