Nearly 400 people, many of them D.C. government employes, braved the high noon heat yesterday to attend a rally to build support for the scheduled Aug. 27 march here in commemoration of the Rev. Martin Luther King Jr.'s historic 1963 March on Washington.

"The question is not 'Why march?' " said Mayor Marion Barry in his remarks to the crowd, estimated at 375 people by U.S. Park Police. "But the question is 'Can we afford not to march?' "

Barry said the "social charter" forged during King's 1963 march "is in serious trouble," and that as long as the federal government spends "$55 billion on something called the B1 bomber" while there is unemployment and poverty, people will demonstrate their opposition.

The rally was staged by the D.C. government to heighten interest in the upcoming march and solicit names of people willing to lend their help by acting as marshals and helpers for the handicapped, said Joseph P. Yeldell, director of the District's Office of Emergency Preparedness.

Many in the audience at Western Plaza in front of the District Building said they were planning to participate in the event, whose theme is "Jobs, Peace and Freedom" and which aims at incorporating many different interest groups.

"Any time you call people together for a common goal is healthy," said Carlene Cheatam, vice president of the D.C. Coalition of Black Gays, which is organizing a gay contingent for the march.

"I'm hoping it will be another way people will realize that we haven't come as far as we could in this time and maybe develop some coalitions. Some of us have done well, but too many are still suffering," Cheatam said.

Michael Arrington, a 30-year-old employe in the city's Department of Transportation, said he recalls that "when the first march took place I was a little kid so I could not participate. I'm going to do my part for the second march to make sure it's successful."

"We're going to find out if King's dream is still alive or not by the number of people who participate," said Leroy Harrison, a printer in the District's General Services Department.

Don Weinberg, director of labor relations for the District, recalled that when King organized the first march "it really was a very inspiring thing and changed a lot of opinions" of whites about the civil rights movement. Weinberg said he plans to take part in the upcoming march.

Meanwhile, support for the march among Jewish groups seemed to be building yesterday with the announcement that the American Jewish Congress, a sponsor of the 1963 march, would endorse the anniversary gathering.

The group, according to its executive director, Henry Siegmand, had initially feared the event might be a forum "for unrelated and divisive issues, including positions that are hostile to Israel and gratuitously adversarial toward this administration."

But in a letter to King's widow, Coretta Scott King, Siegmand said he had been assured the commemoration of King's dream of "a just and compassionate society" would not be used to "chastise Israel, call for recognition of the PLO or the establishment of a Palestinian state . . . or to call for a reduction of U.S. aid to Israel."