If Clinton were to somehow, against all odds, force her way into the nomination, then look for a Nader-type left candidate to really do a number on the Democrats in the fall. It could be Nader, it could be Cynthia McKinney (the latter perhaps especially appealing after Hillary race-baits her way into the nomination), or a combination of both (McKinney is the strongest candidate the Greens have had since Nader, and the Nader/Gonzales campaign is coming out aggressively).
An Obama nomination is substantially less threatened by these tickets precisely because Obama, whether accurately or not (I say not so much) has the leftist credentials that Hillary is deliberately moving away from.
I think it's more likely that a Clinton nomination will spur defection on the left than that an Obama nomination will spur defection on the white...er...right. If Clinton gets the nomination, particularly because it would throw the entire selection policies of the Democratic Party into question, watch a lot of Obama supporters become McKinney or Nader supporters (and at that point I would be happy to say welcome aboard).