NBC Martin Sheen's character, President Josiah Bartlet, is heading into his final year in office on "The West Wing.". Click photo for larger image. Related article: TV Reviews: 'West Wing' fair; 'Universe' poor

When NBC's "The West Wing" has its belated sixth-season premiere tonight -- later than usual so the series can air with limited pre-emptions and no reruns through April -- viewers should brace themselves for change.

Many alterations won't begin until next week's episode as the White House drama moves into the final year of the Bartlet (Martin Sheen) administration with an eye toward the president's legacy and finding a candidate whom Bartlet staffers consider a suitable replacement.

Vice President Russell (Gary Cole), who's running for the Democratic nomination with Will Bailey (Joshua Malina) as his first lieutenant, does not fit that bill. But a new character, a former Houston mayor (Jimmy Smits), catches the eye of Bartlet staffer Josh Lyman (Bradley Whitford) as a man who would carry forward Bartlet's policies. Ed O'Neill ("Married ... With Children") will portray a former Pennsylvania governor who's also a leading contender for the Democratic nomination.

On the Republican side, a California senator (Alan Alda) may be the nominee. And if "The West Wing" continues beyond this season, executive producer John Wells said he's not sure whether he'll install a Republican or Democrat as the fictional president of the United States.

"We have a lot more research and inquiries and talking to do before we do reach a conclusion," Wells said in a teleconference last week.

But first, "The West Wing" has to wrap up May's season finale in which Donna Moss (Janel Moloney) was injured in a Middle East bombing. Tonight she's rushed into surgery for a pulmonary embolism.

"We begin the season dealing with the Palestinian and Israeli issue, which we believe is at the heart of all the problems we're having in the Middle East," said Wells, a 1979 graduate of Carnegie Mellon University (watch for Will jogging in a CMU tank top in tonight's episode). "The episode focuses on possible peace plans, and they're built around plans that were almost approved during the last years of the Clinton administration."

After that, there's some upheaval in the Bartlet White House in upcoming episodes as longtime staffers take on new roles.

"One of the things that happens on a long-running television show, that I didn't know until I did 'ER,' is that unless you shake things up and change the way the characters relate to one another, it's inevitable that you end up with the same character dynamics between everyone. After three or four years, you start to feel like you're repeating yourself."

The changes are also designed to be more in line with reality. Wells said the average White House staffer stays in the job only 18 months because of the burn-out rate.

If Donna survives, Wells said her relationship with Josh may be explored, although for them to move forward something will have to change because Josh is her supervisor and such relationships are strictly forbidden.

First lady Abigail Bartlet (Stockard Channing) will be seen more following the eighth episode of the season when the issue of the president's multiple sclerosis returns to the fore.

This is the final season NBC is obligated to carry "The West Wing." Although ratings are down, particularly among young viewers, Wells said he thinks the series will return for a seventh season in fall 2005, both because of its high rate of viewership among hard-to-reach wealthy viewers and because NBC has fallen on hard times ratings-wise.

"I think losing a show of 'The West Wing's' quality from their schedule this year would be a very bad thing for them in many, many ways," Wells said before playing the role of the 800-pound gorilla TV producer that he is. "And frankly, I do a lot of other business with them ["ER," "Third Watch"] and have been at NBC a long time, so I expect they'll respect the quality of the show and show us respect and make certain we have lots of time to continue to make this show."

If "The West Wing" continues beyond the current season, putting a new president in the White House will necessitate some cast changes, particularly if the new president is a Republican.

This is the final year series star Martin Sheen is under contract. Wells said he'll talk to Sheen at Christmas to see if the actor is willing to return for a seventh season even though his character would leave the White House in January 2006. Wells said he'd like to explore that transition.

"If a new president comes into the White House, what happens to the old president? He's a compelling and complicated character and I'd be interested in episodes about what happens when you leave. What do you do next?" Wells said. "I don't think there's 22 episodes of those, but there are some interesting things to see and explore."

Wells said he'd prefer to have the election take place next season, but if Sheen elects to leave after this year, the show's time line may be sped up to accommodate his departure and the inauguration of a new president.

But the current real-world presidential campaign probably won't have much impact on the TV show, whatever its outcome.

"We have always seen ourselves as an alternative to the existing political situation rather than attempting to immediately reflect it," Wells said. "[We want] to show what our country can be and how our government can be at its best while not trying to be Pollyannaish about the sausage- making that is legislation and government."