Senate Majority Leader Howard Baker (R-Tenn.) last night hosted his annual dinner in honor of the "Republican Majority," an exuberant pageantry of GOP unity that President Reagan seized upon to pitch his defense proposals.

The President and Mrs. Reagan, Vice President George Bush, the big three -- Counselor Ed Meese, Deputy Chief of Staff Michael Deaver and Chief of Staff James Baker -- and at least 50 senators and their spouses roamed the Great Hall of the Library of Congress, hugging and kissing and consorting like only members of the same fraternity can do. "A GOP is 54 strong," said Reagan in his prepared text. "We are still the Senate majority party, and we will remain the majority party."

Noticeably absent from the gathering was Sen. Bob Packwood (R-Ore.), who last week was unseated as chairman of the powerful Republican Senatorial Campaign Committee by Reagan ally Sen. Richard Lugar (R-Ind.).

The Great Hall looked spectacular. Each table had its own Christmas tree centerpiece decorated with gold ribbons and artificial snow. The green trees sat in the middle of red linen coverings, and everyone got a green linen napkin.

The Jefferson Consort, a Marine string ensemble, played 18th-century Christmas music and the U.S. Army Chorus sang popular Christmas songs after dinner. All the women received engraved Jefferson pewter cups from Howard Baker's wife, Joy, recently released from the hospital, who arrived late and was escorted into the dinner by the president.

This year, Baker's party also rolled out the red carpet for the three new Republican senators: Pete Wilson of California, Paul Trible of Virginia and Chic Hecht of Nevada.

"Now you have a senator you can see eye-to-eye with," joked Gail Hecht to Baker, referring to the diminutive size of the two men.

"We together will bring down the average height in the Senate to normal range," said Baker.

"I feel wonderful and delighted," said Wilson, who defeated Jerry Brown in the nation's most publicized Senate race.

Another California Republican, retiring Sen. Sam Hayakawa, was also pretty happy -- but for slightly different reasons.

"I don't like it here," he said.

One senator's date seemed to attract considerably more attention than most of the other guests. When Virginia Sen. John Warner walked in with Barbara Walters, celebrity rubbernecking had its finest hour. The president clutched onto Walters' hands longer than anyone else's, and you could hear George Bush's kiss on her cheek from behind the press ropes. Asked if his appearance with Walters was going to start rumors of a new romance, Warner said, "It's old news, it goes back to 1976."

Reagan, speaking to the crowd, brought up the subject of his MX missile proposal, which was defeated in the House last Monday.

"Now, I won't take this opportunity, at a social gathering, to twist any arms -- much," he said, "but I must tell you now, when we can look each other in the eye, the MX system is needed.

"First and foremost, none of us should forget that the security of America is our highest responsibility . . ."

Earlier in the evening, Baker said the House defeat of the MX "makes it a lot tougher" for the measure to pass the Senate. "It did hurt," said majority whip, Sen. Ted Stevens, of Alaska. "I was thinking the other day, how did Jack Kennedy ever get 47 percent of the budget for defense and today we're having a tough time getting 27 percent? In the long run I think the president will get 90 percent of what he's asking for."

Connecticut Sen. Lowell Weicker, from the liberal wing of the party, wasn't quite as optimistic. "Regardless of how it goes in the Senate, it's not going to make it in passage," said Weicker. "It is up against a determined opposition and there are too many parliamentary possibilities for it to pass . . . I'm against it.

In the true Christmas spirit, the guests dined on roast duck, tomatoes stuffed with wild rice and a fancy Charlotte russe dessert.

"Oh, I think it helps keep us together," drawled Baker, like a proud papa overseeing his flock. "It's good to get together periodically and treat ourselves like a family unit. After all that's really what we are. Right?"