"I'm not really Santa," insisted the roly-poly elf in red jacket and black pants. "I'm a leftover KGB agent."

He could have fooled Nancy Reagan and others at the White House yesterday, where the 1987 Christmas decorations were being unveiled to the press.

More recognizable perhaps as comedian Dom DeLuise, he brought along some summit jokes in his bag of Christmas tricks. For instance, the reason Mikhail Gorbachev got out of his car at Connecticut Avenue and L Street NW last week was because Vice President George Bush, riding with him, had been trying "to rub the spot" (a birthmark) off his forehead.

Mrs. Reagan laughed along with everyone else.

"The president thought he was trying to defect," DeLuise explained.

Mrs. Reagan's annual gig with a mystery celebrity Santa took the press through the festively adorned state rooms, where this year's theme is a musical one. In front of an East Room Christmas tree, the first lady and Santa danced a couple of twirls to a Marine Band tape recording of "Frosty the Snowman."

Instead of having tractors and such things of Reagan Christmases past ("Do you know a mother who's really cuckoo about tractors?" Mrs. Reagan asked), the Reagans' Christmas this year will be "more a mother's Christmas," she said.

That means she and the president are giving each other a carved ivory madonna that Pope John Paul II gave them at the Vatican during their 1982 visit. U.S. law forbids government officials from accepting gifts valued at more than $180 from foreign governments or representatives. The recipients, however, may purchase those gifts, and the Reagans like this one so much they've decided they will.

Elaine Crispen, Mrs. Reagan's press secretary, said the madonna, artist unknown, is carved from a tusk and has been valued at $800.

The Reagans' Christmas dinner guests will be Mrs. Reagan's brother, Dr. Richard Davis of Philadelphia and his family; USIA Director Charles Z. Wick and his family, and consultant and longtime friend Nancy Reynolds and her son.

Asked if friend and former White House deputy chief of staff Michael Deaver, who is being tried on perjury charges, would also be there this year, Mrs. Reagan said, "I don't know."

Mrs. Reagan said her New Year's wish is for yet another U.S.-Soviet summit, this one in Moscow, and further progress on arms control.

She said she wants very much to see Leningrad and the Hermitage if the Reagans go to the Soviet Union. But asked if she wants to tour with Raisa Gorbachev, she paused.

"I look forward to going to Russia," she said.

She refused to be drawn into a discussion of her and Raisa Gorbachev's private cold war, which seemed to have gotten frostier last week.

"I thought that the summit was a very important step, a very important first step," she said. "And there were really serious, substantive things to talk about. All the rest -- I'm not going to," and she never finished the sentence.

As chairman of the National Endowment for the Humanities, Lynne Cheney has worried about the fact that young people don't read the classics of American literature. At the White House dinner for Mikhail and Raisa Gorbachev, she had her worst fears confirmed.

Seated next to Soviet Ambassador Yuri Dubinin, she asked him about the Soviets' interest in the classic frontier novels of Jack London, James Fenimore Cooper and, to some extent, Mark Twain. Whatever other books the Soviets ban, Cheney knew the works of those authors are not among them.

"Fenimore Cooper? I love Fenimore Cooper!" Dubinin replied with great enthusiasm and then added, to her dismay, "But you are the first American I've ever heard say his name!"

Gorbachev's official party consisted of 12 men and two women, so in seating the Reagans' dinner there was no way to avoid putting two men together at several tables. Mrs. Reagan, following protocol, sits next to the guest of honor, and Reagan next to the visitor's wife at another table. The question was whom to put on the other side of the Soviet leader. The White House says it was Nancy Reagan's decision to seat Rep. Richard Cheney (R-Wyo.) there.

"She knew he was highly respected and a good conversationalist," said a White House official.

Cheney is also unflappable and that afternoon when the White House alerted him where he'd be sitting, he took his placement in stride. A member of the House Intelligence Committee, Cheney has followed arms control carefully. Which was just as well. As an aide to Cheney said later, "Gorbachev wasn't interested in the usual state dinner small talk."

The National Symphony's Mstislav Rostropovich thought Van Cliburn's playing for the Gorbachevs so wonderful that he should return immediately to active performing. "To deprive the public is a sin," he said.

The symphony's irrepressible maestro, in black tie like his host and all the men except the predominantly male Soviet delegation, was equally fascinated by the Soviets' shabby suits. As he stood watching his former countrymen walk by, he turned to Armand Hammer.

"Armandchik, you love the Soviet Union," Slava said to this Kremlin intimate since the days of V.I. Lenin. "You must make them a gift of tuxedos."

George Shultz, the State Department's host-with-the-most, left nothing to chance when he planned his State Department luncheon for the Gorbachevs. He ordered an all-American meal of Maine lobster, Maryland crab, Alaska king crab legs, Louisiana shrimp and Pennsylvania venison.

Only when it came to the apple pie did Design Cuisine, the caterer, need Mom in the kitchen.

It took Design Cuisine's French chef five tries before coming up with one as delicious as the kind Helena Shultz has been baking for her husband all these years.

"I could make them," she volunteered at one point during the preluncheon taste tests.

"Yes," said Chief of Protocol Selwa Roosevelt, "but you can't make 25!"

White House staff photographers Mary Ann Fackelman-Miner, Pete Souza, Bill Fitz-Patrick and Susan Biddle shot about 300 rolls of film during the Gorbachevs' three-day visit. The number of pictures taken wasn't so unusual, says White House photo editor Carol McKay. What made the Gorbachevs' visit different was that they could take their album of more than 50 pages of color pictures home with them. Usually the time lag between last picture taken and completion of the album is about two weeks, and the albums are sent to heads of state through diplomatic channels. This time, the White House photo office worked virtually around the clock.

The Wyeth name rang a bell when Nancy Reagan mentioned it at the White House Wednesday morning by explaining that Pat Nixon's official portrait was the work of Henriette Wyeth.

"Wyeth, Wyeth?" Raisa Gorbachev repeated. "The whole generation is very impressive."

The works of three other Wyeths, Henriette's father N.C., her brother Andrew and nephew Jamie, went to the Soviet Union last winter in an exhibition titled "An American Vision: Three Generations of Wyeths." Mrs. Gorbachev never got to see the exhibition, but author Suzanne Massie managed to get a show catalogue to her and Mikhail Gorbachev.

That Raisa was aware of the show was apparent at Shultz's lunch for the Gorbachevs, where Raisa sat between the secretary of state and Rep. Tom Foley (D-Wash.). Also at the table was Andrew Wyeth.

After talking to Raisa for a while, Foley, who isn't majority leader by accident, noted that she seemed particularly well read in contemporary American literature and interested, generally, in the cultural world. Seeing a chance for her to converse with one of the country's foremost contributors to that world, Foley suggested that he and Wyeth trade places so the artist could talk to Raisa.

"It seemed the appropriate thing to do," said a spokesman for Foley.