The old upright piano plinked out a Protestant hymn standard and the senior citizens who had gathered for a government-subsidized lunch raised their voices in familiar verse: "I am weak but thou art strong, precious Jesus lead me on . . . "

David Clarke, a D.C. City Council member who once studied to be a preacher, sat comfortably among them, slowly rocking his 6-foot-5, 250-pound frame in time to the music as steaming plates of baked chicken and scalloped potatoes were brought from the kitchen.

"I almost want to preach," said Clarke, as he began his speech on why he wants to be chairman of the City Council and why the elderly and infirm people of Capitol Hill Towers, 900 G St. NE, should care.

Schooled in the social activism of the 1960s and with eight years of legislative activism on the council, Clarke painted a bleak picture of cuts in the federal budget and social programs sought by the Reagan administration and said the council may "have to provide what Ronald Reagan takes.

"I am the only one of the candidates who has stood for something. Not just that everything be good and nothing be bad. I have signed my name to legislation and stood behind it," said Clarke, citing one of the themes of his campaign to win the Sept. 14 Democratic primary over incumbent Arrington Dixon and former chairman Sterling Tucker.

In a race that political observers say is too close to call, Clarke is taking the biggest political gamble of his career, giving up his secure seat on the council from Ward 1 to make the uphill bid for chairman.

There is no runoff -- a plurality, however slim, wins the nomination -- and no Republican to face in the fall.

In an early opinion poll, Clarke, who is running his first citywide council campaign, trailed both Dixon and Tucker in voter recognition, registering well only in his own central-city ward that includes Howard University, Adams-Morgan and Mount Pleasant, Le Droit Park, parts of Shaw and Kalorama.

Clarke, 38, is stressing his willingness to tackle such controversial issues as gun control, condominium conversion curbs, rent control and antispeculation penalties.

As chairman of the judiciary committee, he played a major role in revamping the city's criminal code and narcotics laws.

He proposed the first bill to eliminate the sales tax on food and introduced the bill to tax homeowners at a lower rate than for commercial property.

Because of such activism, some of his supporters urged him to run for reelection to his council seat this year, rather than risk defeat and being out of office altogether.

"My feeling was that it was better to have Dave Clarke just a simple council member than not to have Dave Clark at all," wrote Sam Smith, publisher of the D.C. Gazette, a political tabloid.

Smith since has endorsed Clarke for chairman.

Some council members and political observers point to that same record of activism and wonder whether Clarke can make the transition to the chairman's demanding role of mediation and management.

Critics and even some of his supporters have faulted Clarke for being quick to become angry or frustrated during council debates.

"I am an intense person," Clarke concedes.

Several council members who have worked with Clarke said he has tried during the past year to quell his temper.

At the same time, Clarke tries to capitalize on general criticisms that Dixon has failed to encourage cooperation among the 13 members of the council.

"I will listen to the council members," Clarke said at a forum sponsored last week by a Ward 7 professional and business group.

He pledges he will meet regularly with the members and "give the council a sense of where it is we want to go."

Clarke is viewed warily by some real estate and business interests in the city, largely because of his support of such housing issues as rent control and restrictions on condominium conversions.

To counter such views, Clarke has stressed to audiences his support for the new D.C. Convention Center, job training programs and reductions in unemployment and worker compensation costs to city businesses.

He recently proposed granting tax credits for contributions to neighborhood community development organizations, many of which are losing federal funding.

Clarke also is noted for sometimes allowing legalese to mangle his speech, throwing in such words as "condominiumization" and references to the "chairmanic race."

Despite his history of involvement in the civil rights movement, including work for the Southern Christian Leadership Conference in Washington and the 1968 Poor People's Campaign here, Clarke, who is white, must contend with the political fact that he is trying to win the second-highest elected city government office in a city that is 70 percent black.

Earlier this year, Clarke initially decided against challenging Dixon, who is black, partially because he said he feared that racial distrust had worsened due to the policies of the Reagan administration.

Clarke got back into the chairman's campaign when no other candidate announced against Dixon. Tucker, who also is black, was a surprise late entry.

"I'm not trying to pretend I'm black," Clarke said last week when asked about the effect race may have on his campaign. "The things I do affect me as well as anyone else. I don't do it from a messianic perspective that I'm trying to save somebody. It fits my value sense to do it."

Clarke's campaign received a boost early yesterday morning when he was endorsed by Bishop Walter (Sweet Daddy) McCollough, the influential head of the predominantly black United House of Prayer for All People.

A resident of Washington since moving here at the age of 2 with his mother, Clarke says his father died in World War II.

Clarke lived in the Shaw neighborhood for 20 years, staying with an aunt after his mother died when he was 16. He attended the city's public schools and graduated with a degree in religion from George Washington University. He holds a law degree from Howard University. His wife, Carole, is a teacher in the city's schools and his son is a student here.

In 1966, Clarke, then 22, was a member of the Free D.C. Movement, a group that demonstrated for home rule in the District of Columbia. That year, Clarke joined a group of members who went to protest at the annual Cherry Blossom Festival Ball being held at a D.C. hotel.

The peaceful protest landed Clarke and five others in jail while another young home rule activist named Marion Barry, who helped establish the Free D.C. Movement, read a statement in support of them outside the hotel.

"The council in my mind is an extension of the civil rights movement," says Clarke, but he adds, "The methods are different, extremely different. The movement stressed confrontation while the council should work through cooperation."

He laid the groundwork for his campaign for chairman over several months and received the private encourgement of Mayor Marion Barry, who has feuded with Dixon over budget issues.

Working out of his rambling home-turned-headquarters on 17th Street in Mount Pleasant, Clarke has put together a largely volunteer organization that has won the respect of his opposition. Clarke has raised about $70,000, just under half of Dixon's reported $150,000 but more than Tucker's $50,000.

Clarke has used the money for extensive leafleting and scores of campaign posters and has sought to bolster his support within neighborhood and activist groups. With his low budget, he also is trying to make the most of free television and radio appearances. He has taped a number of 30-second radio commercials that will air later this month.

Clarke has won more endorsements than Dixon or Tucker, including nearly unanimous backing from public and private sector unions.

Clarke also has gained the endorsements of such groups as the D.C. Women's Political Action Caucus, the local chapter of the Americans for Democratic Action, the Washington chapter of the National Black Women's Political Action Caucus, several tenant organizations, the Gertrude Stein Democratic Club and the Ward 6 Democrats.