From Holden:

The Pony Effect is kicking in.

“He has no political capital,” said Tony Fabrizio, a Republican pollster. “Slowly but surely it’s been unraveling. There’s been a direct correlation between the trajectory of his approval numbers and the — I don’t want to call it disloyalty — the independence on the part of the Republicans in Congress.”

[snip]

With Bush’s approval rating ranging from 34 percent in a CBS News poll to 41 percent in the latest Washington Post-ABC News survey, some Republican candidates facing the voters in just eight months worry privately that, unlike in 2002 and 2004, he will be more albatross than advantage for GOP candidates in the fall campaign.

UPDATE: Latest from AP-Ipsos : 36

Chris has some juicy bullet points.