Responding to my post, Dave Weigel offers an explanation for why conservatives remained solidly behind Bush until at least 2006:

1) War. There was no serious conservative opposition to Bush from September 11 2001 to some point in 2006. The Medicare Part D vote was held two months after the Iraq War began. When your base supports you on a war, you can get away with some disappointments on other issues. Barack Obama simply doesn’t exploit his commander-in-chief role the way Bush did. (If you think that’s unfair to Bush, I have video of a certain pilot landing on an aircraft carrier you should see.)

2) Winning/Losing. The Medicare Part D vote was sold to conservatives as a more market-friendly version of a Democratic idea, which would take their idea off the table for the 2004 election. The tax cut deal looks like sad president bowing to Republican obstruction in the Senate and giving in to “hostage takers.” Bush’s successful feints to the left were always sold as ways to grab voter-friendly Democratic ideas to benefit Republicans. Democrats don’t see what they’re getting with the tax deal — it affirms, as Jim DeMint says, what conservatives spent 10 years saying about tax cuts.

These account for some of the reasons why Bush did not face a rebellious conservative movement until the 2007 immigration debate. The degree of uniformity and lockstep support for the administration that the Iraq war created does help explain why Bush could run much more to the left than Obama has moved right without provoking a backlash. The war largely dominated the politics of 2004-2007, Bush and the movement were united on the same side, and outside Washington the movement and his party’s rank and file were the only ones who were still with him when he proposed the “surge.” It is possible that the sheer unpopularity of both Bush and the war forged a connection between Bush and conservative war supporters that would not have otherwise endured. As long as the war remained the defining issue, it bonded the movement to Bush to an extent that has never happened with liberals and Obama. Conservatives might reflect on that and consider whether waging an unnecessary, costly war that harmed U.S. interests was worth the massive expansion of the welfare state before and after 2006 that it enabled.

As the movement became convinced of the success of the “surge” and Iraq faded into the background, the movement became a bit more willing to challenge Bush when they found his policies unacceptable. Obviously, many of Obama’s core constituencies haven’t responded to Obama’s escalation in Afghanistan in anything like the same way because many of them are ambivalent or openly opposed to the decision. Where support for the “surge” became the single-most significant litmus test on the right, Obama’s decisions on foreign policy and national security have generated mixed reactions at best on the left because of significant continuity of many Bush and Obama policies.

The timing of the two episodes also makes a big difference. When Bush pushed through the prescription drug benefit, it wasn’t just that he was making it seem like a Republican win on a traditional Democratic issue, but he was also still relatively popular both within and outside his party. He was coming off the unusual post-9/11 “Khaki” election of 2002 in which his party gained seats, and there will still illusions among some activists on the right that an enduring Republican majority was there for the taking. Indeed, according to the fashionable ideas of some “big-government conservatives” and “compassionate conservatives,” it was supposed to be the prescription drug benefit and things like them that would make the GOP into the dominant party in the future. All of that proved to be false, but it wasn’t immediately clear that it was false. At the time, it was tempting for some on the right to see it as a major partisan victory. Obama’s capitulation comes on the heels of a major electoral defeat and resulted from Democratic avoidance of the tax issue until after the election. Despite the best efforts of David Brooks and Charles Krauthammer, it is difficult to spin this as success. Instead of theoretically poaching on the other party’s territory, it represents a policy setback coming in the wake of a significant political setback. It also serves as confirmation in the minds of liberal critics that Obama will not “fight” and will accommodate the opposition, whereas Bush’s gargantuan entitlement expansion was more or less in keeping with what conservatives had come to expect of Bush. Perhaps one of the most important differences is that many of Obama’s liberal allies see the deal as a failure of leadership, and Bush’s conservative allies saw the prescription drug benefit as evidence of successful leadership, albeit in the service of expanding the welfare state. This is probably why the reaction to the tax deal is as strong as it is despite the comparatively smaller concessions that Obama made.