It wasn't such an unusual thing, really: a state's congressional delegation holding a Washington reception for a prospective new congressman -- a reception attended by lobbyists, civic leaders and key legislators.

What made last Tuesday's reception special was that the sponsoring group comprised the entire Mississippi Democratic congressional delegation and that the honoree, introduced by the governor of the state, was black.

Robert Clark, who recently won his party's nomination for Congress, was cool, observing dispassionately that his nomination -- and the makeup of the reception crowd -- was proof that "Mississippians, white and black, can unite in a common bond to tackle the problems that confront us."

Joe Rauh, a guest, was ecstatic, calling it "one of the happiest nights of my life." "So many people feel we haven't made any progress in this country," he said. "Well, this is a celebration of progress."

The venerable civil rights leader, who first met Clark in 1964 at the old Masonic Temple on Lynch Street in Jackson -- at a session that gave birth to the Mississippi Freedom Democratic Party -- couldn't suppress his astonishment at the change. One of a handful of whites at the 1964 meeting, Rauh remembers how dangerous it was for him even to be there. "I just kept telling myself that if I get killed for this, it's for a good cause."

Last week he marveled at the sight of Sen. John Stennis and other Mississippi politicians waiting in line for a chance to be photographed with Clark--photos that, in earlier years, might have been the stuff of blackmail but, on this night, were being rushed back to Mississippi for use in the state's newspapers.

The glad-handers included members of the civil rights establishment and the Black Congressional Caucus as well as lobbyists for the sugar and cotton industries. Clark (from rural Holmes County in the Mississippi Delta) has said he plans to seek a seat on the Agriculture Committee, whose chairman, Rep. Kika de la Garza (D-Tex.), was on hand to greet him.

Rep. David Bowen, who is retiring from the newly reorganized 2nd Congressional District that is Clark's home base, thinks Clark has a very good chance to succeed him: "The latest poll shows him with a 10-point lead (over Republican Webb Franklin), including 20-some percent of the white vote. Another 17 percent are undecided, but the very fact that so many white people are undecided is important. It means that there is not sufficient hostility that they will vote against him simply because he is black."

Bowen said he recently walked with Clark down Greenville's Washington Avenue, the main street of the district's biggest town, and was encouraged by the response of white merchants.

"People, a lot of whom voted for Reagan, were telling us that they thought the president had tried to move the country in the right direction but that they thought we had gone far enough in that direction and maybe it was time for a swing back to the Democrats. They were impressed with Clark, once they met him."

Although Clark's district is 54 percent black, the consensus is that it will take a white vote of at least 10 percent to elect him. His effort to garner that vote can only be helped by the support of Stennis, Bowen, and Reps. Jamie Whitten and Sonny Montgomery, all of whom attended the reception. Bowen says the help goes both ways. The presence of Clark's name on the ballot will increase the turnout of black voters, overwhelmingly Democratic, and boost the reelection chances of the 81-year-old Stennis, he explained.

Clark, a 15-year veteran of the Mississippi Legislature, said the across-the-board nature of his support shows "how far Mississippi has come in putting the issue of race behind us."

"During my 10 years in Congress," Bowen said afterward, "I prided myself on the fact that I was able to represent all my constituents, white and black. I think a lot of us think it important to understand that Clark will do the same thing. It's about time. The last black member of Congress from Mississippi took office in 1883."