American politics today is not just about winning elections or prevailing on issues. It's about delegitimizing, or preventing the delegitimization of, our presidents.

This thought sprang into my head as I was reading the angry and sometimes obscene Democratic Web logs and noted the preoccupation of some bloggers with the impeachment of Bill Clinton, now seven years in the past. For them this, was a completely illegitimate exercise, because Clinton was being attacked for his sex life. I think this is wrong, since reasonable people could either (a) say Clinton deserved impeachment because he lied under oath in a federal court proceeding or (b) say that impeachment was inappropriate, because the offense was not central to his service in office.

Naturally, most Republicans agreed with (a), and most Democrats with (b): We all tend to break ties in favor of the home team. The Democratic bloggers note correctly that impeachment didn't help the Republicans politically. But they still seem incensed, and I think that's because they believe that impeachment, in their view unfairly, tended to delegitimize the Clinton presidency.

It has been a habit of presidents to try to write their own history, to establish themselves as a legitimate embodiment of America's past and shaper of America's future. Franklin Roosevelt did it better than any other 20th century president, relating his actions to those of Thomas Jefferson, Andrew Jackson, and Abraham Lincoln, his cousin Theodore Roosevelt, and his onetime boss, Woodrow Wilson.

FDR encouraged the idea that history is a story of progress toward an ever larger and more generous government. That version of American history was propagated by a brace of gifted historians and in most mainstream media. For decades afterward, presidents were judged by the FDR standard. Harry Truman was crude and ineloquent, but he made tough decisions and got them mostly right (a view that stands up well). Dwight Eisenhower smiled and played golf, but was an inarticulate bumbler (a version that doesn't stand up at all).

John Kennedy and Lyndon Johnson were stalwart and compassionate liberals (which ignores the facts that they embarked on the Vietnam War and wiretapped Martin Luther King Jr.). Richard Nixon was a villain who left in disgrace (even though he extricated us from Vietnam and greatly expanded government). Ronald Reagan wrote a different version of history. Like FDR, he showed that a strong and assertive America could advance freedom in the world. But unlike Roosevelt, he saw government at home as the problem, not the solution, and he utterly refuted the liberal elites who said that low-inflation economic growth was no longer possible and that America was on the defensive in the world.

Not so. We've had low-inflation growth for most of the past 25 years, and the Soviet Union has disappeared. History doesn't always move left -- sometimes it moves right. Democrats unsurprisingly don't like this version of history, and in Bill Clinton they had a president with the articulateness and political instincts to offer his own. He could claim that his policies, like Reagan's, produced prodigious economic growth and that his limited military interventions promoted freedom and democracy.

But impeachment cast a pall on his record, and so did September 11: Clinton (like George W. Bush in his first eight months) failed to address what turned out to be a deadly threat. Bush's version of history is mostly in line with Reagan's. Since Sept. 11, he has led an aggressive policy against foreign enemies, while lowering taxes and pursuing, with considerable success despite narrow Republican majorities, mostly conservative policies at home.

Democratic politicians and the mainstream media, who bridle at the Reagan version and are disappointed that it has not been displaced by Clinton's, regarded Bush's victory in the Florida controversy as illegitimate and have been trying furiously to delegitimize him ever since.

So far, this has proved at least as ineffective politically as impeachment was for the Republicans, but the impulse to persist seems irresistible. How long will this continue?

Democrats were used to writing our history in most of the past century. But without a competing vision of their own, they seem no more likely to succeed than Roosevelt's or Reagan's furious opponents.