In one corner of the Convention Center ballroom, denims and boots were doing a "cakewalk," walking around a square and stopping on a lucky number to win a cake. On center stage, dancers in rainbow-colored tights were tapping and kicking as though they were the Washington Rockettes.

On the sidelines, youths in cowboy hats--plastic, straw and felt--were lined up to throw darts at balloons.

This was Del. Walter Fauntroy's midway, a county fair in the city's mammoth, concrete meeting hall. Last night's fair was his constituents' hearty salute, on his 50th birthday, to the District's only representative in Congress.

"Now Walter, now that you're publicly celebrating your 50th birthday, would you please stop telling people that we went to school together?" laughed Marilyn Brown, a former D.C. school administrator, as she kissed Fauntroy and the brims of their cowboy hats locked. Brown and Fauntroy attended Virginia Union University together. "We go back a long time and he's the same sweet Walter."

The question of whether a county fair can be genuine without pig-greasing, manure, bales of hay or blue-ribbon heifers didn't bother many of last night's 500 paying guests. "On Wednesday we decided to have wood fences around the booths because the fire insurance to bring hay bales inside was too costly," said Lezli Baskerville, a lawyer, who was last night's coordinator. The fair theme was chosen, she explained, "because we wanted to do something to involve a broad-based cross-section. The ticket prices wouldn't be prohibitive."

His children had never seen a fair, but the Rev. William Bynum had attended plenty in Wilson, N.C. "Out in the country that's all they have," he said. "And this one brings back some memories." By the shooting gallery, former D.C. police chief Burtell Jefferson was aiming precisely at his target. "We've never had a real fair around here. I'm a city boy," he said.

When Fauntroy, who has been the city's nonvoting representative in Congress since the position was created 12 years ago, was asked to join the square dancing, he said, "Since I am a square, I'm going to try." Most of his evening was spent successfully shooting baskets, signing record albums that people won (not his album) and listening to constituent complaints. His actual birthday was Feb. 6, but scheduling didn't permit a celebration until last night. The opening of the Convention Center and winning the right for the Convention Center to bid on rock concerts were the highlights of Fauntroy's last year.

Among those having their fortunes told or eating hot dogs were Benjamin Alexander, president of the University of the District of Columbia; David Clarke, chairman of the D.C. City Council; council members Hilda Mason (Statehood-At large), Betty Ann Kane (D-At Large) and William Spaulding (D-Ward 5); Ruth Bates Harris, the former executive director of the D.C. Office of Human Rights, and the Monk family, a group of young men who sing, play the violin and read poetry.