Last September, I noted this graphic from Barry Burden at Harvard, which suggests approval of President Bush drives the approval ratings of Congress:

The latest issue of Time includes details from a leaked memo suggesting that Republicans agree:

In an internal Republican Party memo provided to Time, Jan
van Lohuizen, a longtime Bush pollster, warns candidates tempted to
distance themselves that “President Bush drives our image and will do
so until we have real national front-runners for the ’08 nomination.
If he drops, we all drop.” Another Republican strategist describes
the problem for g.o.p. candidates this way: “Adding weight to the
anchor doesn’t help them.”

This is what economists call an externality. The president’s approval affects the fortunes of the whole party, but his incentives are different than those of members of Congress. So you see these ugly splits where party members want to distance themselves from unpopular presidents like Bush but fear turning the public further against the leader of their party. It’s a nasty problem with no good solutions.