Within their own parties, right now, McCain is out-performing Barack by about 6 points, scoring 76% of all Republican votes compared to only 70% of Democrats for Barack. That's about to be expected considering that McCain has had a couple of months now to consolidate his base while the Democrats have been through a bruising primary period. Most of this difference is attributable not to cross over votes (McCain wins 17% of Democrats and Obama wins 16% of Republicans) but to undecided - 13% of Democrats are undecided compared with only 8% of Republicans.
Obama is winning by 3 points amongst independants, 15% of whom are at this point undecided. But remember that with these number as they stand - and these numbers will clearly improve for Barack as the primary comes to an end - Obama wins by 8 points!
Now look at the breakdown by ideology. McCain wins 78% of conservative voters, and Obama wins 71% of liberal voters. So far this matches up with the party ID stuff (although who are the 18% of self identified liberals who say thei will vote for McCain? That's just nutty). But now look at "Moderates" - Obama dominates this category by 56% to 33%. This is great news for the general election, and also suggests that the attempts to make Obama look like an extremist through playing up the Reverend Wright affair etc. have not penetrated with this group at all. So far.