Willie Nelson unhooked his guitar from its red, white and blue macrame straps. He walked forward to the lip of the stage and raised both hands in a hug-salute to the cheering crowd at the Merriweather Post Pavillion Saturday night. He then turned around to face the side of the stage, where Jimmy Carter was clapping along with the rest of the crowd.

Nelson grabbed the president in a bear hug so tight it was hard to tell where the singer's dark-T-shirt ended and the president's dark sport shirt began. Of course, it was helpful to know that Willie Nelson's head was the one with the long braids.

The embrace was the highlight of the Willie Nelson & Family benefit concert for the Carter-Mondale campaign.It was the second time Nelson had donated his services to Carter this year; the first was at Atlanta's Fox Theatre in April. In return, Carter invited Nelson to the White House Friday. They went jogging together, and Nelson spent the night.

Carter is an unabashed Willie Nelson fan. Originally, the president was scheduled to spend only 20 minutes at the show, but he personally switched an evening speech to the afternoon so he could stay for the end of Nelson's performance. After their bear hug, Nelson disappeared, but Carter stayed on stage to clap for an encore.

Willie Nelson is not the typical performer befriended by politicians. Not many singers have entertained heads of state with songs about bar fights, cocaine, seduction and unrepentant murder.

Nelson's friendship with Carter is as unusual as was poet Robert Frost's with John Kennedy. Like Frost and Kennedy, Nelson and Carter share a regional identification. Nelson and Carter's rapport, however, goes deeper than the Southern drawl they both speak with. It goes back to the church.

Nelson gave the spiritual, "Will the Circle Be Unbroken," a rousing finish. Then he said, "I'd like to invite the president to come up and help us sing 'Amazing Grace.'" The TV floodlights blazed on and Carter slid out of his row and walked up on stage.

The president obviously needed no prompting to remember the words of the old hymn. Nelson and Carter were joined by Amy Carter, Frank Moore, Carter's congressional liaison, and Nelson's daughters, Paula Carleen and Amy Lee. It was a family church choir, just like back home.

After the song, Carter sat down next to the lighting board as if he were just another roadie. The president clapped along to the next hymn, "Uncloudy Day," as if he were sitting in a wooden pew instead of on a steel equipment trunk.

Paul English, who has played drums for Willie Nelson since 1954, said he didn't mind being called away from vacation to donate his time to the Carter Campaign. "First all, we're having fun. Secondly, whatever Willie wants to do is fine with us. I support President Carter, but I don't see how we can do him any good. We're a real motley group of friends." English was wearing gold-tipped red cowboy boots and sunglasses jewel-embroidered with a dollar sign and his initials.

Across the service road from the long-haired cowboy musicians was a green and white striped tent where the big donors sipped coctails. They paid $500 a seat; the general public paid $15 a seat or $10 to sit on the lawn. p

One of the donors was Joe O'Connell, a Silver Spring businessman and longtime Democratic contributor. "When someone asked me to contribute $1,00 for a Willie Nelson?' They said he was the guy who didn't know the words to 'The Star Spangled Banner' at the convention," said O'Connell.

Up on the pavilion's hill, those in the cheap seats had different motivations for coming.

"I'm a John Anderson supporter myself," explained Duane Stewart of Bowie. "I hope they lose my money."

"It wasn't until we went to Ticketron to buy the tickets that we found out it was for Carter," said Coralee Herbst of Springfield. "I haven't slept since. I'm a Republican. I just wanted to see Willie."

How did she feel about Carter coming on stage? "Carter wasn't here!" She exclaimed. "You're kidding! You mean the president was here? That's what we get for sitting in the nickel seats."