That question is explored in depth in an excellent new article by Farhad Manjoo in Salon. Here’s an excerpt, but the whole article is worth reading:

Bush’s second-term agenda was so unapologetically bold — he wanted to privatize Social Security, flatten federal taxes, remake the courts and, on the side, democratize the world — it bordered on the revolutionary. In November, as liberals were sunk in the delirium of defeat, their in boxes buzzing with comic maps dividing North America into the United States of Canada and Jesusland, it seemed that nothing could rein the Republican president in.
Six months later, Bush is the dog that didn’t bite. He approaches the end of the first 100 days of his second term with approval ratings that fall below those of all other reelected presidents in the modern era. Americans aren’t happy with the direction in which the country is heading. They don’t like the economy, and they don’t like the war. They also don’t like Bush’s plans for the nation. If it isn’t already dead, Bush’s signature domestic-policy effort, the plan to privatize Social Security, is in a persistent vegetative state; hated by Democrats, independents and even Republicans, only divine intervention can save it.
Now the question is whether Bush’s sinking popularity — and his desire to stick with the unpopular Social Security plan — will hurt the Republican Party’s agenda over the next two years and beyond. The GOP continues to advocate world-changing plans. Conservatives want to amend the Constitution, alter the Senate’s rules on judicial nominees, and disrupt long-standing fiscal, environmental, global and social norms. At the same time, Bush looks boxed in. There’s no money in the federal till to implement his tax cuts. The military’s stretched too thin for him to invade another country (such as Iran). And the federal courts are holding his social agenda in check.
Some key Republicans are beginning to balk at Bush’s extremism. On questions involving the Social Security plan, or the details of the federal budget, or the confirmation of Bush’s nominees, a few moderate Republicans have begun to go against White House plans. If the American public continues to turn away from Bush, political strategists say, it’s only logical to expect more defections from their Republican representatives on Capitol Hill.