Although the Obama campaign likes to claim he's won over Clinton supporters, the fact is he hasn't. Polls done of people who voted for Hillary reveal only a small percentage of those who said they would vote for McCain back in March, April, and June before he secured the nomination have changed their minds. Just consider that 28% of Hillary supporters said they would vote for McCain in March, 24% of them said they would vote for McCain in April, and 23% of them said they would vote for McCain in June.
Almost nothing has changed. A poll conducted in the last few days by NBC put the number at 21%, and another 27% are still undecided. That means only a quarter of those who said they would support McCain have changed their mind, and the undecided category has actually grown by at least that much and possibly more.