The Concert was over, but balladeer Harry Chapin was still energized, if hoarse, from the two-and-a-half hour show. "Well, felt good," he shouted huskily draping his arms around Chip and Caron Carter for a photographer's benefit. Then, after an exchange of small talk, and, in what was meant to be a stage whisper [you get the feeling that Chapin is incapable of whispering]: "I'm going out to politic a little bit" - and he strode off to work the crowd under the trees, the prophet in the wilderness.

Well, not really the wilderness. One corner of the Merriweather Post Pavilion grounds had been roped off and a buffet set up. In keeping with the general spirit of less is more, it was appropriately spare - as befitting a benefit for the Food Policy Center meant to call attention to world hunger - if artfully arranged to look rather more than less.

The guests, including Senators Howard Metzenbaum (D-Ohio) and Patrick Leahy (D-Vt.), a host of congressmen and the Carters, stood around under the are lights, swatting insects and snacking on cheese and fruit. And trying to keep up with Chapin.

He moved through the crowd, calling hellos in his roughened voice and spouting statistics and explanations. As Father Bill Ayres, the Roman Catholic priest from New York who first involved Chapin in hunger issues in 1973, said, "We try to move people in little steps, from A to Z."

"A is you're uninformed; maybe you don't care.And Z is, well, Harry is Z."

Chapin, known for songs like "Taxi" and "Cat's in the Cradle," has been "Z" for five years now, and Thursday's Merriweather Post concert and attendant party was planned to celebrate the pinnacle of those years, the creation of the President's Commission on World Hunger. But as Leahy, the Senate sponsor of the resolution founding the commission, said yesterday on phone, "It probably would have been going by now, but Peter Bourne was handling it in the White House and . . ." and you could hear him shrug.Still, Leahy said the commission should probably begin work next month.

Chapin has a novel approach to the commission's business. "Right now, on the Burmese border, there are 140,000 children starving," he said, dropping the tone of his voice for the first time in the evening and speaking seriously, "and I think that would be a good place to hold a commission hearing."

"Hiya, Harry" said one young man in a Jimmy Buffett T-shirt. Everybody calls him Harry. "You may not remember me, but once when you were at Marquette University I drove you to the airport, and I'd like to talk with you about this hunger bit."

"If you could wait just a second," replied Chapin. "I'd love to talk with you." And turning back, he extended his criticism to the West. "I'm not only talking about villages in Bangladesh; I'm talking about villages like Chicago!" he said.

But the fact that he was able to get something started no one more than in the peculiar village of Washington pleases Harry Chapin. An old friend, he stayed at Leahy's house here, said the senator, and in between sing-alongs organized for Leahy's three children, they sat and discussed the forthcoming commission, of which Leahy will be a member. "Some performers, they show up for the press conference and make statements to the media, and that's the last you see of them," said Leahy, "but Harry's a day-after-day guy:" And Chapin gives more than time to the hunger crusade - by his own count, the hundred crusade - by his own count, the hundred or so benefits he did last year earned the Food Policy Center more than $700,000.

His diligence has gotten him the ear of the right people - Chip Carter's presence at Thursday's party demonstrated his concern for the isues Chapin helped turn him onto, and also a concern that the commission doesn't get overlooked.

Taking time out from a busy, election-year schedule at the Democratic National Committee, Carter laughed - "It's tough. They want me to do fund-raisers for candidates and I failed speech twice in college" - and sipped white wine while he talked about his involvement and about Chapin.

"We worked with him last year on the hunger stuff," he said, "and I saw him in concert and invited him to the White House. He's a real good guy, and this has really got me interested."

As Carter turned away to move his small family through the bustling crowd, stopping to talk with Leahy about the similarities between driving in Vermont snow and on wet Georgia clay, Chapin was still haranguing the crowd.

"What we're trying to do is raise an attitude about hunger in this country the way there was about slavery in 1850. There's no economic basis for hunger, just as there wasn't for slavery - to put the moral issues aside for a minute. But there's a vacuum, hungry people don't have power," he is telling a small group of enraptured listeners.

And finding his way through the corridors of power has been an education for Chapin. "You've got to take the time to deal with the staffs," he said, and grinned. "I learned it in the record business - the secretaries run everything." Today, in people like Leahy's estimation, Chapin is becoming a pretty good lobbyist.

"Anybody can get on TV. Squeaky Fromme got on the cover of Newsweek for pulling a pistol on President Ford. I went to the people who make the laws, and I came back. They said I'd never get anywhere - eight months later I was sitting there with Pat Leahy and Dick Clark and Rep. Rick Nolan (D-Minn.), the House sponsor of the resolution, and I was speaking for Congress!"

The party was winding down, Merriweather Post security guards sat yawning in a golf cart, waiting for the guests to leave - and most had. There was Harry Chapin, still energized and reexplaining his program and the Food Policy Center to the 20 or so people still left, taking special time to talk with the kid from Marquette in the Buffett T-shirt.