On that score, Mr. Sheen was offered an opportunity to see how his character's appeal would play in a real-life campaign. Not long ago, he said, he was approached by Democratic Party representatives from his native state, Ohio, to see if he would be interested in running for the United States Senate after he left the show. Though he would have had little trouble drafting a campaign platform -- he is a fierce opponent of nuclear power and the war in Iraq, and a champion of human rights -- he turned them down.

"I'm just not qualified," he said. "You're mistaking celebrity for credibility."

Nonetheless, Mr. O'Donnell, a onetime adviser to Senator Daniel Patrick Moynihan of New York, said he was especially proud of the show's response to the increasingly shrill political debate in the real world, particularly on cable news. As it became tougher to learn much of any substance from programs like "Crossfire" on CNN, now defunct, "The West Wing" seemed to delve deeper into real issues like health care and education, as exemplified by the raw, one-hour live debate last fall between Matt Santos and Arnold Vinick.

"Political talk on TV has degenerated so much," said Mr. O'Donnell, who is also a political analyst on MSNBC. "You can say something complex on 'The West Wing' and you will not suffer a screaming interruption by three other panelists."

It may not come as any surprise to viewers, given that President Bartlet was a Democrat, but there were no registered Republicans in the most recent incarnation of the "West Wing" writers' room, which included Eli Attie, a former speechwriter for Al Gore. Though the show began at the end of the Clinton administration, it soon found its creative niche by evoking a parallel reality, one that imagined how the White House might have been different if George W. Bush had not been elected to two terms.

As the war in Iraq escalated, Mr. Sheen said he came to liken the show's role to that of good, escapist fiction.

"In order to sometimes get a different perspective on what's going down in the world, to reach back to your humanity, you read novels," Mr. Sheen said. "We're like the reading of a novel."