On March 8 at the Washington Convention Center, Mayor Marion Barry plans to give what he calls a "nonpolitical, nonpartisan, nonsectarian" 50th birthday party for himself -- a milestone for the city's two-term mayor, and probably also the last nonpolitical event of this election year.

Although the mayor remains a heavy favorite to run for and win a third term this fall, in the view of most District elected officials and political activists, the conviction of his longtime political confidant, Ivanhoe Donaldson, has introduced a wild card into the contest.

Potential challengers who would not have considered entering the race six months ago are calculating their chances.

U.S. Attorney Joseph E. diGenova's ongoing corruption investigation, which sent Donaldson to a federal penitentiary this week on fraud and cover-up charges, has given encouragement to such potential mayoral candidates as City Council Chairman David A. Clarke and council members John Wilson (D-Ward 2) and Charlene Drew Jarvis (D-Ward 4).

"There is an undercurrent of discomfort out there," said Joslyn N. Williams, president of the Metropolitan Washington Council AFL-CIO and a longtime Democratic Party regular.

Williams said, "Folks are wondering what else is out there. What does Ivanhoe know? Is he or isn't he going to be talking? If he talks, what is he going to say? Who will he implicate?"

Also encouraged are Barry's only announced challengers, former school board member Mattie Taylor and publisher Dennis Sobin -- though neither has the kind of established base the council members would have, with Sobin's campaign seen by most as frivolous and Taylor's viewed as serious but lacking in support from well-known local Democrats. As usual in the District, with its overwhelming Democratic majority, attention is focused on the Democratic primary rather than the general election.

But despite what many Barry watchers now are concluding is discernible damage done to the mayor by the Donaldson conviction, Barry's political wounds are not viewed as fatal. The reluctant and the devoted appear to agree that, in the words of restaurateur and Barry supporter Stuart Long, "Marion is unbeatable."

"He can raise so much money and take so much money out of this city that another candidate wouldn't be able to run a formidable campaign," said Long. "He has been pretty much unscathed, even though people close to him like Ivanhoe have been. He has been hurt, but he hasn't been touched."

Barry, who the business community says could easily raise between $1 million and $2 million for a reelection campaign, brashly has issued a challenge to any potential candidate.

"If I do run, I want to take on all comers, including David Clarke," Barry said in a recent interview. "I understand he's out talking about [running]. I want a strong race. I want the government to be scrutinized."

Clarke, meanwhile, said he made between 100 and 200 telephone calls to determine how much support he would have for reelection as council chairman. In the process, he said he received a number of unsolicited requests.

"A number of people have suggested that I should run for mayor," said Clarke, who declined to name the individuals. "I asked them what is the dissatisfaction [with Barry], and more importantly, can you tell me what you want from me if I were to do that in terms of a positive program . . . . I am listening to what they have to say but I am not focusing on running for mayor."

Even without Donaldson in his camp, Barry still has the best political organization in town. According to Long, Barry is putting together his reelection finance committee, and the mayor has indicated that if he does launch a campaign this year, Anita Bonds, a key worker in his previous campaigns, will run it.

In 1982, Barry formally began raising funds in January for the September primary, in which he trounced challenger Patricia Roberts Harris. But Bonds said that a final decision about Barry's 1986 candidacy will not be made until after his March birthday party.

Meanwhile, strategies of political timing in the mayor's and the City Council chairman's races are being plotted behind the scenes, though no one is yet willing to make a first move. Several scenarios have been envisioned:

*Barry may wait to announce in April, forcing Clarke to decide between running for chairman or entering the mayor's race before Barry. If Clarke runs for mayor, the chairman's race would be wide open and likely attract a large field of contenders. In that case, if Clarke encountered difficulty in the mayor's race and opted to drop out, he would have a hard time winning an 11th-hour bid to retain his current seat.

*If Clarke or Wilson or Jarvis enters the mayor's race, the other two might view that as a signal that Barry is vulnerable and jump in as well, creating a crowded field that could produce an easy Barry victory.

*Jarvis, viewed as potentially a strong challenger to Clarke, may enter the chairman's race and trigger Wilson's entry into that race. The result would be a three-way race similar to the 1982 chairman's race in which Clarke prevailed over former council chairmen Sterling Tucker and Arrington Dixon.

*Clarke may not risk a campaign for mayor because, with his term ending, a defeat would leave him without a political office. On the other hand, Wilson and Jarvis are in the middle of their four-year council terms and would retain their seats if defeated by Barry.

Although Clarke, Jarvis and Wilson are actively testing the waters to see how much financial support they could muster, their unwillingness to make their intentions public and begin building an organization suggests to some that they are merely flirting with the idea.

"I would think that in addition to getting money, people would be putting together an effective citywide organization," said John R. Tydings, executive vice president of the Greater Washington Board of Trade. "It is tough to run a citywide race and win."

Mattie Taylor is trying to form a citywide organization and concedes that it is tough. There is a reluctance among many leaders in the city to take on the powerful mayor, she said.

"I spent all summer calling leadership people in the city to find a candidate I could support [for mayor]," she said. "Nobody would run and nobody could tell me of anybody who was running. I decided to do it myself."

Taylor and any other candidate who enters the race faces not only an incumbent with a strong political organization but one whose liabilities are overshadowed by his perceived success in keeping the city prosperous. The fraud conviction of Mary Treadwell, Barry's ex-wife, and the cocaine distribution conviction of former D.C. employe Karen Johnson, a personal friend of the mayor, would have more seriously damaged a less popular politician than Barry.

James Christian, chairman of the D.C. Democratic Party, said that most people he has talked to are sympathetic to the mayor and believe that he has had no direct link to the Donaldson case or other malfeasance cases.

"I think overall the city is in great shape," he said. "We are not severely economically depressed. We don't have major racial tension or conflict. There are no major symptoms of great social ills here in Washington. I think he is getting credit for having a very healthy city."

Still, there are pockets of disaffection that a challenger could capitalize on. In largely white Ward 3, which rejected Barry in the 1982 primary, Barry's chances have been hurt by the Donaldson episode, said Jim Nathanson, who is seeking the Democratic nomination for the ward's council seat.

"I would say that the mayor was behind in Ward 3 and probably before Ivanhoe he had a chance to break even," Nathanson said. "But frankly, I think at this point he would have to devote a lot of time to Ward 3 and Ward 3 interests to recoup."

"The effect of [the Donaldson case] is harmful," said the Rev. A. Knighton Stanley, pastor of the Peoples Congregational Church. "The question is, is it enough to defeat him? The answer to that is no. We know that from the word go and it doesn't make you feel good about government."