It was the least a mother could do. So Coretta Scott King, widow of the slain civil rights leader, found herself not only at Washington's new Polo Club last night, but also posing for pictures with the rhythmic soul group Earth, Wind & Fire.

"I'm pretty tired," she said from behind half-closed lids. "But when Marty made his final plea, I said okay."

"Marty" is Martin Luther King III, son of the civil rights leader. Last night he was appearing as co-founder of a group with the not-so-snappy title of "The Commission for the Advancement of Policy Affecting Youth, the Disadvantaged, and the Poor."

The Commission for the Advancement etc. was holding a reception for Earth, Wind & Fire because that very hot group promises to lend its support (not financial, just moral and verbal) at concerts across the country.

"My brother talked me into it," said Maurice White, mastermind of the group, who was besieged upon arrival by Instamatics and autograph hounds.

THE owners of the Polo Club were delighted they had attracted yet another crop of names to their night spot. Said Lee Nathanson, one of the four investors: "I can't believe I'm walking around carrying Coretta King's pocketbook."

The 300-plus crowd that listened to a tape of "Boogie Wonderland" was predominately young, black, trendy and well dressed.

"I went to the Congressional Black Caucus fashion show on Saturday, and they said 'Ladies, it's feathers,' so I went straight out and bought feathers." This from Lorraine Westbrook, who works for a "very high government official" and wore bronze sequins on her dress and, you guessed it, bronze feathers in her hair.

White House press aide Jo Carpenter wore a slinky black gown and lots of gold jewelry, prompting club owner Tom Curtis to marvel at the result. "She said it only took her 20 minutes to get ready," he said, telling her she nonetheless could have arrived in her 9-to-5 work clothes.

"You think I'm a schlemiel or something?" responded Carpenter.

Also popping in from the White House was Steve Selig, assistant to chief of staff Hamilton Jordan.But Selig left early and missed most of the action, the fault of a parent-teacher conference he had to attend at the Sidwell Friends School.

Transafrica lobbyist Randall Robinson was talking politics, and mentioned he'd met with Secretary of State Cyrus Vance last week about Zimbabwe-Rhodesia and what he calls insufficient U.S. aid to African nations.

And how did Vance respond? "I can only say that the meeting was constructive and the discussions were useful," said Robinson.

That was about it for political conversation. "Have you heard about this new razor for black men?" a young woman at the bar asked her companion. He hadn't.

Mayor Marion Barry arrived late to present White with a service award for his support of the commission, which co-founder Raymone Bain said is still "in the incubation stage."

Coretta King said a few words, then left. "I'm not especially a fan of what -- do you call it? -- rock music," she had sighed earlier. "I'm going to go home to Atlanta in the morning and try to get all the things cleared off my desk."