Michael Barone, one of the best political analysts, writes a very interesting summary of the primary election season to date for the National Review Online. He notes that neither McCain nor Obama is the overwhelming choice of the voters in his own party. Barone being Barone, he gives you the numbers to prove his contention.
He points out that both Obama and McCain have benefited from the mistakes of their competitors. McCain essentially hoping that none of the other candidates would "catch fire" with the electorate, and they didn't. Obama's lead in pledged delegates a result of Clinton's failure to organize in the caucus states.
McCain is not terribly popular with conservative Republicans and Obama is not terribly popular with blue-collar Democrats. We could possibly see cross-over voting, ticket-splitting, and/or maybe voting "no" by staying home. We might even see non-trivial numbers of Democrats voting for Ralph Nader and Republicans voting for Bob Barr. These, by the way, are my conclusions, not Barone's.