The contenders for the D.C. Democratic mayoral nomination, seeking to shore up support with a key constituency, kicked off the final week of primary campaigning yesterday at the annual Labor Day Mass at the Shrine of the Sacred Heart.

The Mass represented a last-minute opportunity for the five Democratic hopefuls -- John Ray, Charlene Drew Jarvis, Walter E. Fauntroy, David A. Clarke and Sharon Pratt Dixon -- and other candidates in the Sept. 11 primary to press the flesh with the city's top labor leaders and other politically active union members.

More than 400 people attended the Labor Day program, which featured a traditional Catholic service with a sermon delivered by Catholic University President William Byron and remarks by John J. Barry, president of the International Brotherhood of Electrical Workers.

It concluded with the laying of a wreath by Mayor Marion Barry and other politicians at the statue of Cardinal James Gibbons outside the church at 16th Street and Park Road NW.

The mayor, who has kept a low profile since he returned last week from a vacation in Jamaica, declined to discuss his candidacy for an at-large seat on the D.C. Council. "I want you all to cover the mayor's race," Barry told reporters.

Jesse L. Jackson, fresh from a trip to Iraq, put in a brief appearance at a reception after the service, resuming his campaign for one of the two shadow senator seats to lobby for D.C. statehood. In the past few days, hundreds of posters have gone up across the city, heralding "Jackson: U.S. Senate" under his photograph.

Also at yesterday's service were several candidates for the D.C. delegate's seat, including Sterling Tucker, Joseph P. Yeldell, Donald M. Temple and Eleanor Holmes Norton, who is scheduled to kick off a $63,000 television campaign this morning, the first in that contest.

The city's estimated 75,000 union members, many of whom are government workers, are among its most politically conscious residents, and the labor vote could be a decisive factor in the Democratic primary next Tuesday.

Because the District's labor leadership has split its support among the five Democratic mayoral contenders, the candidates are now taking their messages directly to rank-and-file members.

For example, Dixon has been shunned by the major city unions, which are leery of her slash-the-bureaucracy message, but she argued yesterday that she is well-positioned to capture labor votes in the primary. "This is the group that's most eager for somebody to cut that upper-level management bloat," Dixon said, citing one of her major campaign promises.

Dixon also played down her dearth of endorsements from major labor unions, saying, "You're talking about the same old power brokers, whom I think are out of touch with the rank and file."

Jarvis pointed to her endorsement from the American Federation of Government Employees, the D.C. Nurses Association and other local unions.

She also noted that the Committee on Housing and Economic Development, which she chairs, recently approved a bill that would expand benefits to injured workers under the city's workers' compensation program.

"I feel very confident, very frankly, that the votes that the labor community can bring to me are going to be the votes that will make me mayor," she said.

Clarke sought to address doubts that he can win the primary because he is the only white candidate in the race.

In a brochure distributed yesterday and to be sent to 20,000 labor households this week, Clarke contends that "some have chosen to overlook what they admit to be Clarke's impressive labor record and endorse others in the mistaken belief that . . . he cannot win in a city which is predominantly black."

"I think the rank and file will support me just fine," Clarke said yesterday. "They're not worried about some of the things their leaders are worried about."

Ray, who has been endorsed by the local police, firefighters and teachers unions, asserted that he and Clarke have the best labor records of the candidates running and questioned whether Jarvis or Fauntroy would be able to capitalize on their union endorsements.

"It is going to be very difficult for the unions who have endorsed candidates who don't have good labor records to lead their people anywhere," Ray said.

Fauntroy, who left the service early and was not available for comment, has been endorsed by the local hotel and restaurant workers and several other prominent unions and is counting on the unions for a big get-out-the-vote effort on Election Day, his advisers say.

Staff writer R.H. Melton contributed to this report.