(Vernon Bryant/Associated Press)

There’s no doubt that the president can focus both the public and the political system on a particular issue. Iraq is the ultimate example. Neither Congress nor the American people were thinking very much about Saddam Hussein and Iraq in July 2001, or even in October 2001. By the middle of 2003, it was all they were thinking about. That’s the power of presidential agenda setting. It is, perhaps, the most significant power the president possesses.

But it, too, is a limited power. In particular, it’s limited, at least in my view, by the fact that presidents are typically unable to persuade the public or the Congress of things they don’t already believe. This is what keeps the president from being, in political scientist George Edwards’s terminology, a “director of change” and makes him instead a “facilitator of change.”

So the question with Iraq is whether it’s an example of agenda setting, persuasion or both?

The polling suggests that although Iraq was a remarkable example of ambitious and sustained agenda setting, was not necessarily an example of persuasion. John Sides posts this graph from Gary Jacobson:

(Gary Jacobson)

That doesn’t take away from Bush’s achievement, such as it was. He saw that 9/11 presented an opportunity to invade Iraq, and he was very effective in using his agenda-setting power to make that happen. If not for him, the fact that Americans were passively in favor of regime change in Iraq wouldn’t have mattered. But if Americans hadn’t been against regime change in Iraq — if he hadn’t been working off of a favorable base line — it’s not clear he could have succeeded.

To put it another way, it sounds obvious to the point of being silly to say that if Bush had asked Congress to authorize war with Peru, they wouldn’t have done it. But that’s actually precisely the point: There was a preexisting willingness to “finish the job” in Iraq, and that was crucial to Bush’s success. If the “persuasion” lift had been larger, the war likely wouldn’t have happened.

That said, you can see something else I argue in that graph: Bush’s efforts on Iraq eventually polarized public opinion. In 2001, large majorities of Democrats and Republicans supported further action against Hussein. By 2004, only 20 percent of Democrats supported the war.

Kevin also argues that Ronald Reagan’s presidency changed the public’s attitude towards taxation in an enduring way. This is conventional wisdom, but it’s not evident in the polling. If anything, the belief that the income tax people paid was “too high” fell after Reagan: