Gallup reports that 28% of Democrats who support Clinton claim that they would vote for McCain if Obama were the nominee.
A sizable proportion of Democrats would vote for John McCain next November if he is matched against the candidate they do not support for the Democratic nomination. This is particularly true for Hillary Clinton supporters, more than a quarter of whom currently say they would vote for McCain if Barack Obama is the Democratic nominee.
good news ?!?!?! yes, because we know about overall support for McCain vs Obama (they are roughly tied). If Gallup is to be believed, the support for McCain in national polls currently includes 28% of Democrats who support Clinton. In the actual election, it is extremely unlikely that so many of them will actually vote for McCain.
Thus the news from the Gallup poll is not that Obama has a problem (we know he has a problem he isn't ahead of McCain in the polls) but that, it appears, that a significant part of his problem is something that is likely to get worse before it gets better, but get better before election day.
McCain is not likely to get 28% of the votes of Democrats who support Clinton, therefore he is not likely to get as many votes as suggested by current national polls.