Wow: McCain leads among likely voters in new Gallup poll; Update: Young voters sour on Obama?

And to think, I figured today’s happy poll news would be that Rasmussen sees Obama’s post-speech bounce on Thursday and Friday as having disappeared over the weekend. You want disappearances? According to a Gallup tracking poll taken Friday through Sunday, i.e. after the “We Are the World” singalong in Berlin, Obama leads by eight among registered voters. But according to, er, another Gallup poll taken over the same period, Obama’s lead is only three — and he actually trails by four among likelies. Even better:

The Friday-Sunday poll, mostly conducted as Obama was returning from his much-publicized overseas trip and released just this hour, shows McCain now ahead 49%-45% among voters that Gallup believes are most likely to go to the polls in November. In late June, he was behind among likely voters, 50%-44%.

As for the difference between the tracking and USA TODAY/Gallup polls, Frank [Newport, chief Gallup pollster,] says not to read too much into it. “Statistical noise” may be largely to blame.

The bad news: The eight-point lead is based on a much larger sample (2,600 versus 800) so it’s probably more reliably. As for why McCain is suddenly up among likelies, Newport suspects a backlash among conservatives “energized” by Obama’s European trip. But why would that number be larger than the number of liberals excited by all the photo ops with world leaders plus the number of centrists who doubt whether Obama’s ready for prime time feeling reassured by the fact that his trip went relatively smoothly?

Update: Brian Faughnan points to a piece of data noted last week by Michael Barone that may explain this: The percentage of adults 18 to 29 who say they’re likely to vote has dropped 20 points since March. But why?