If Romney somehow magically gets the nomination, then I think a strong 3rd party candidate might jump in on the right, and honestly, I wouldn’t be suprised to see one jump in on the left too, especially if Obama’s numbers go lower than 40 and stay there.
I agree on both counts. On the Democrat side, their base won’t vote for Romney, but they might just stay home. A fourth party liberal challenger gets them to the polls, preventing (they hope) a Republican blow-out down ballot.