District Mayor Marion Barry redefined the two-party system last night, following up a 50th birthday party earlier in the week for deep-pocket political contributors with a roof-raising nonpolitical birthday party last night at the Convention Center that drew a throng estimated at 5,000.

The two-term mayor, having announced he will delay his official reelection campaign announcement for another eight weeks at least, worked the crowd energetically and appeared to relish the atmosphere of what could have passed for a coronation.

"Happy Birthday," Calvin Rolark, a Ward 8 publisher and community activist, told the mayor, embracing him amid a clot of people who gathered early at the party. Barry, asked by one how his campaign was going, replied: "What campaign?"

Later, on stage, after Washington rock band Radiant had sung "Happy Birthday" to Barry, the mayor told the crowd: "Tonight is a night for fun. This is nonpolitical, nonpartisan, nonsectarian, a nonannouncement -- except you all know where I'm going."

About 80 "patrons" contributed $500 each, nearly 700 "sponsors" kicked in $50 apiece and thousands paid $15 to attend -- enough, organizers said, to foot the estimated bill of $100,000. Leftover proceeds are expected to be turned over to charity.

Guests who bought their tickets at the door were asked to leave their names and addresses. Betty King, one of the organizers of the event, said that "in politics there's nothing as valuable as a list."

Party-goers listened to Dixieland music, standing in groups as the party got under way. Table seating for 2,500 had not yet been filled. But organizers were optimistic that the largest mayoral birthday party ever was about to take place.

"There's been a birthday party for the mayor ever since 1978," said Janet Dewart, head of the firm that was hired to stage the party by a nonpartisan group called Friends of Marion Barry. "It was thought that, this being his 50th birthday, there should be a big to-do about it. A loose-knit group of tight-knit friends decided that they would have a bigger party than we've had before."

Last month a few dissenting voices piped up to complain that the mayor's 50th birthday party should not have been ruled a nonpartisan event by Keith Vance, director of the city Office of Campaign Finance. To the sour strains of those who argued there is no such thing as a nonpolitical $100,000 party in a campaign year, Vance replied that Friends of Marion Barry does not have to file financial reports with the campaign finance office.

City Council member John A. Wilson (D-Ward 2), a possible challenger to Barry in this year's Democratic mayoral primary and one of the few council members who declined to be named an "honorary patron" for the event, termed the ruling "disgusting."

Other council members who steered clear of honorary patronage were Betty Ann Kane (D-At large) and Carol Schwartz (R-At large), but the rest of the city's legislature and a host of other political lights were so designated, including Del. Walter E. Fauntroy and 11 school board members.

The crowd at the Convention Center last night included persons who paid $15 for their tickets and who might have missed two major events earlier in the week that were aimed at the city's upper-income set.

On Thursday, Citizens to Reelect Mayor Barry staged a $1,000-a-plate breakfast at the Washington Hilton hotel to kick off the fund-raising drive for Barry's expected campaign. More than 125 turned out. The next night Barry shook hands with members of a much larger crowd at the Greater Washington Board of Trade annual dinner.