http://election.msn.com/?GT1=10928
Even if Clinton quits, which I don't think would happen, McCain will probably win. That would include people who vote for McCain, people who vote for a third party or do a write-in, and people who opt not to vote at all because they are discontent with the candidates and process.
Obama is doing extremely well in the primaries but that doesn't mean that voters are ready for Obama (the change that he claims to stand for) and that election day will come out in his favor.