At the top of the staircase there arose such a clatter that the media looked up to see what was the matter.

Shouting "Ho, Ho, Ho, Merry Christmas" was no ordinary elf. With Nancy Reagan on his arm, down the stairs came Santa himself.

That might be the way a modern poet would describe how the holiday season began yesterday at the White House, and yes, Washington, there is a Santa Claus.

"I'll give you a few hints," the first lady said of his identity. Like her, he was clad in a seasonally stylish red suit. "He likes pigs, you shouldn't ask him to lift anything heavy and he's very happy today."

Reporters who recognized the bright blue eyes and Kansas twang needed only one guess to come up with the name of Washington Redskins running back John Riggins. He put an early present under everybody's Christmas tree Sunday when he scored the winning touchdown and led the Redskins to a 30-28 win over the Dallas Cowboys.

"We had to win it," Riggins said from behind his long white beard and peering out from under his false white eyelashes, "or we wouldn't have been able to be here this morning."

While Mrs. Reagan said she wants a pickup truck for Christmas (that, coincidentally, is what President Reagan also wants though Santa expressed some doubts about getting it down the chimney), Riggins said he would like "a little trip to Palo Alto" where the Super Bowl will be played.

If Santa's belly, which Mrs. Reagan gave a couple of kindly pats, failed to live up to expectations, it wasn't for lack of trying, according to Riggins.

"What I should have done was eat more, but it was like a Catch-22. The coach didn't think that would be a good idea, " he said, standing in the Blue Room where the 20-foot blue spruce Christmas tree from Michigan took up the rest of the space. At another point he said it had been "a lean year at the North Pole. Santa needed a little bit of help. He had a bunch of elves walk out in the middle of a busy season."

Riggins spent last week hospitalized and in traction for treatment of a bad back. Recovering sufficiently to play on Sunday, he said yesterday his back was "just a little sore -- not too bad."

Riggins, who met the president yesterday morning, said Reagan told him that he hadn't been able to watch the game or read the sports pages. But White House social secretary Gahl Hodges said she had watched the game with considerable anxiety. "I knew who our Santa was going to be," she said, "and I was sitting there saying to the TV set, 'Don't let anyone beat up on John Riggins.' "

Mrs. Reagan said the 2,800 Christmas ornaments on the huge tree were made by young people from Second Genesis, a residential drug treatment program in the Washington metropolitan area, and by volunteers at the Brandywine Museum in Chadds Ford, Pa. Using such natural materials as pine cones, nuts, dried thistles and milkweed, they fashioned tiny animals, stars, angels and other figures.

When someone asked why there were no toys under the tree, Mrs. Reagan looked surprised.

"There aren't any usually before Christmas ," she said.

"I still got all that out on my sled. You can't watch me at work," said Riggins, whose wife Mary Louise and three children, Portia, Robert and Emil, accompanied him.

"That's my Santa," Mrs. Reagan said, visibly relieved.

In the East Room, six more blue spruce trees were decorated with dried baby's breath flowers, tiny lights and glass icicles. The Cross Hall was an explosion of poinsettia trees, pine boughs, red bows and Christmas wreaths, one of which had supplanted Richard Nixon's new official portrait on the south wall.

In the State Dining Room was assistant executive chef Hans Raffert's 36-inch-high gingerbread house. Illuminated red, blue and green windows (made of gelatin leaves) represent the Red, Blue and Green rooms at the White House.

Accompanying Santa and Mrs. Reagan on this part of the tour was Lucky, the Reagans' 9-week-old black sheep dog, decked out in a red and green knitted cap with the words "Christmas 1984 -- The White House."

Taking it all in was the White House correspondent for the official Soviet news agency Tass, Alexander Shalnev, who told Mrs. Reagan he thought the decorations were "very beautiful."

"I'm glad to have you here," the first lady said.

Earlier, she told reporters, including Shalnev, that what she wants for Christmas "and I suppose what everybody wants, is peace."

Shalnev was seen scribbling something in his notebook.

About 650 are expected at the Reagans' big Congressional Ball tonight, but one who is not is Rep. Geraldine Ferraro (D-N.Y.). She'll be in Madison, Wis., under the auspices of Time magazine, addressing a University of Wisconsin audience on the future of the Democratic Party.

Ronald Reagan was the first in Time's Distinguished Speaker series. Ferraro is the second and in the view of some she is also a prime candidate for Time's annual person of the year cover.

The party was so casual that yesterday morning the hostess still hadn't decided what she was going to serve. But when you've been friends as long as they have, as Carrie Lee Nelson said of the dinner she and Gaylord Nelson gave last night for former Senate colleague Walter Mondale and his wife, Joan, "You don't just get together for Carrie Lee's chicken breasts."

The invitation to a "preholiday lunch" was surprisingly formal. And, indeed, once guests assembled Sunday at La Chandelle Restaurant the surprise was on them.

The hosts: Faith Collins, former deputy press secretary to Rosalynn Carter and now executive assistant to the head of Fannie Mae (Federal National Mortgage Association), and Thomas J. Scanlon, founder and president of Benchmarks, Inc., a consulting firm that specializes in international development assistance. They were married the day before by the Rev. Theodore Hesburgh of the University of Notre Dame, who flew in especially to tie the k; t.