Stacy brings up an important point: President Bush has left many Senate Republicans between a rock and a hard place. Blue-state incumbents are getting hit by their Democratic challengers for voting with Bush on Iraq, health care, and the economy. Red-state incumbents are frequently in trouble with the conservative base for voting with Bush on amnesty and the bailout.
Of course, Senate Republicans are hardly blameless themselves. Since the Contract With America, they have been undercutting their more conservative House colleagues. Just in recent years, they have pulled out the rug from under the House GOP on immigration, energy through the Gang of Twenty, judges through the Gang of 14, marriage through failing to bring up a House-passed jurisdiction-stripping bill that was backed by the Bush administration, and the bailout. That's one of the reasons it has been so difficult to get conservatives to focus on this year's Senate races, even though avoiding a filibuster-proof Democratic majority is a more vital conservative objective than the outcome of the presidential contest.