After dinner, the singing began. Richard Nixon and almost 200 former staffers and friends, all collected at the Washington Marriott Hotel on Saturday night for a 10-year reunion, held hands and sang a slow, nostalgic version of "God Bless America." Then Nixon himself, more relaxed and loose than some said they'd ever seen him, took a seat at the piano and played it again.

And that wasn't all. Once the crowd had gathered around the piano, the former president asked, "Is it anybody's birthday?" It was, so he played "Happy Birthday" with gusto. "Don't stop!" urged the crowd. Nixon, like a radiant life of the party, obliged. His selection? "Let Me Call You Sweetheart."

It was the end of an evening of laughs, a few tears, some leftover bitterness and, from the faithful, unflinching pride. Watergate was never mentioned -- except by reporters -- but it seemed an invisible thread, a war that forever bonded its veterans.

When the dinner was over, making news as parties rarely do, guests came out from behind the ballroom doors to tell what happened. Nixon, they said, told the crowd that during his administration, "all these people put all this effort in -- and they didn't get a chance to finish the job." But, he added, they "can still have a role of leadership." The room, decorated with ficus trees strung with tiny white lights, repeatedly was filled with cheers and applause. "I am just emotionally drained," one guest said afterward.

Rose Mary Woods, Nixon's former secretary, ended the evening. Guests described her as saying in the final toast: "Richard Nixon is the most honorable man this country has ever produced."

The 10-year reunion, celebrated just after the anniversary of Nixon's landslide election, was a whole weekend of activities organized by his old White House advance team. It was chiefly for the advance staff, the group that made all presidential travel arrangements. But the dinner was the highlight, so Nixon and his former staff were invited. Many hadn't seen each other since they'd been at the White House; some were there after serving time in jail for Watergate crimes. Nixon himself was making one of his infrequent appearances at a large social event; his wife, Pat, was unable to attend.

H.R. (Bob) Haldeman and John Ehrlichman, Nixon's top two, couldn't make it either. But former attorney general John Mitchell and former secretary of state Henry Kissinger did. So did aides Gordon Strachan, Dwight Chapin, Charles Colson, Pat Buchanan, Steve Bull, Egil (Bud) Krogh, Ron Ziegler and Jerry Warren. They all turned up in black ties and tuxedos, hugging each other and kissing the women.

They only talked about Watergate when asked. "You know why?" said Warren, the former deputy press secretary. "That's over, as far as this group is concerned. For them to be burdened by Watergate is a moral imposition. Those who have been in prison are now out, and they shouldn't suffer again."

There also was a lot of cocktail party wisecracking between members of one of the world's most notorious fraternities. Ron Ziegler, the former press secretary, saw a good chance when he spotted his former deputy. Warren, now the editor of the San Diego Union, was waiting for a drink.

"Look at this," said Ziegler. "The editor of the San Diego Union is leaning over the bar."

"Ron," said Warren. "You never were this loose before. What happened?"

"Well," said Ziegler, "I have a good staff now." The Great Escape

Nixon wouldn't speak to reporters before the dinner, but there seemed some indication he might venture out and say a few words after the dinner, which reporters were kept from. Nixon's aides thoughtfully set up velvet ropes near one of the ballroom doors. Reporters and camera people took up position, their lenses and ballpoints poised.

Nothing much happened for half an hour, although hotel guests wandered by and waited, their own cameras strung around their necks. Suddenly there appeared Ed Nixon, Nixon's younger brother. Some comments from a next-of-kin seemed like an interesting possibility, so reporters gathered around. Ed Nixon was in fine spirits as he talked about his brother's remarks to his guests.

"He was very, very, let's see--how would I say it? He was looking at the future, as he has been lately. He expressed the hope that . . . our country must prevail, that the Western World as we know it . . ." Suddenly, emerging from the ballroom, three doors and perhaps 50 yards down:

Nixon!!

But it was too late. A few reporters gave chase, but Nixon had already hopped onto the moving escalator with his security men. Hotel guests might have been amused by the scene: There was Nixon, waving like a confident candidate on a moving escalator (and almost in tune with the hotel piano player) as reporters hung over the balcony and watched him being carried away. The Voice of Experience

Pat Buchanan, Nixon's former speechwriter, was in a great mood. "I interviewed the old man for an hour today on CNN," he said. In the interview to promote his new book, Nixon said that Ronald Reagan shouldn't compromise on the third year of the tax cut, but that Secretary of Defense Caspar Weinberger should "bite the bullet" and make some defense cuts.

There were four Reagan administration staffers at the dinner; five had been invited. Those attending were Craig Fuller, the Cabinet secretary; Joseph Wright and Lawrence Kudlow, both of the Office of Management and Budget; and Mort Allin, the national director of "Youth for Nixon" in 1967 and 1968. Now he works in the White House press office. Several of those invited, including Fuller and Kudlow, had been part of a team who'd briefed the 1972 advance people on the economy and foreign policy in the Old Executive Office Building earlier Saturday. Lee Atwater, from the White House political office, was invited but didn't come.

The current White House staffers said they didn't feel awkward among the Nixon folks. "We were home this weekend," said Kudlow. "It seemed like a nice thing to do." Fuller offered the same rationale. But when Allin appeared at the party, Fuller exclaimed: "There's another high White House official! I'm not the only one!" Speeches & War Stories

Much of the dinner, which included beef tournedos with gulf shrimp and blue baked-meringue elephants with raspberries and whipped cream for dessert, was taken up by speeches. Ron Walker, the former chief of the advance team and the main organizer of the event, introduced Nixon, who reportedly said, "Thirty-two years ago, when I was in Taft, California, I could never have gone anywhere, even the bathroom, without an advance man." Then, referring to Kissinger's work in his administration, Nixon said: "We couldn't have done it without Henry."

"None of us," Kissinger responded later, according to guests, "including myself, could have done it without the leadership of Richard Nixon."

Then Nixon played the piano, and was gone. But the guests stayed on, listening to members of the advance team. They took turns at the podium, telling the old war stories from 10 years ago. By midnight, they were just warming up