Chris Bowers critiques the flip-flop line of attack against Barack Obama and observes "In fact, looking at the national poll trend lines at Pollster.com, the only line of attack that has ever clearly damaged Obama was the Reverend Wright flap back in March." Indeed as you can see above that's true. I might add that Wright-based attacks, while in many ways unfair, had the proverbial "grain of truth" advantage in that I think Obama really is more liberal than the Democratic nominees of the recent past in a way that trying to associate him with radical figures drives home.
The interesting thing about the 2008 election is that the political marketplace has responded to the collapse in support for Bush and the GOP in a pretty efficient manner -- with the Republicans nominating someone who's somewhat less conservative than Bush and whose association with the GOP brand is relatively weak, and the Democrats running on a more liberal agenda than they've had in recent cycles. McCain wants to exaggerate how moderate he is and how left-wing Obama is, not recycle attacks on John Kerry. I bet most people wish Kerry had won (actually, it would be interesting to see polling on this).