When the clock struck midnight last night, the changing of the old guard at the Senate became official, and for the Democrats there, bitter reality. But senators from both sides of the aisle gathered yesterday evening, with families and friends, to honor two men who have served for many years as the true guards of that body.

Secretary of the Senate Joseph S. (Stan) Kimmitt and Sergeant at Arms F. Nordy Hoffmann are two casualties of the switch to a Republican Senate majority, and the showing of guests at their lavish farewell reception was proof of the respect they have garnered during their tenure as the chief administrative officers there.

In his remarks to the crowd, lameduck Sen. Gaylord Nelson (D-Wis.) threw an aside to Secretary of State Edmund Muskie, who was also present to give best regards to Kimmitt and Hoffmann.

"If you don't do a better job of getting Nordy out of office than you did getting the hostages back," Nelson ribbed Muskie, "he'll still be here when Reagan leaves office."

Nelson got more laughs from his audience of Senate insiders as he roasted Kimmitt.

"Stan always presided over our meetings with an even hand," he said. "Whenever our wives or members of our staff called, he regularly and convincingly denied any of us were present."

Muskie quoted Winston Churchill as he noted that most of those who showed up to eat crab claws and salmon in the grandiose Senate Caucus Room were of the Democratic persuasion, the party which nominated and elected Kimmitt and Hoffmann to their positions.

"Churchill said of one of his political opponents, 'He has all of the virtues I dislike and all of the vices I admire,'" Muskie said. "This is a Democratic gathering, and we've got a right to have a Democratic party when our numbers are diminishing . . . As we slip into an unaccustomed role, I hope we'll not yield to becoming an irresponsible or demagogic minority."

Vice President Walter F. (Fritz) Mondale, looking sunburned in contrast to the secretary of state's rather pale face, spoke after Muskie made an emotional presentation to Hoffmann of a pair of fishhook cuff links the sergeant at arms had given him in 1962. Mondale joked and his audience laughed on cue.

"One thing [Kimmitt and Hoffmann] did, even though they came up in the Democratic Party and owe their election to the Democrats, was that they never failed to deal fairly with members of either party no matter who they were or how contemptuous they were," Mondale said. "And if you believe that, I've got some costume jewelry to sell you."

Hoffman came to Washington as a lobbyist for the United Steelworkers Union. He went on to head the Democratic Senatorial Campaign Committee before going to the Senate. But the one thing everyone remembered last night was his time under Notre Dame football coach Knute Rockne. Sen. Ernest Fl Hollings (D-S.C.) introduced him to the group as "the all-American Gipper."

"If someone had told Knute Rockne I'd have been sergeant at arms, he'd probably have had them put away for good," Hoffmann said.

Kimmitt came to the Senate, after leaving the Army as a colonel, as administrative assistant to Sen. Mike Mansfield (D-Mont.) in 1966. He said he will continue to work in Washington "with a single company." He said the thing he remembers above all is the long wrangling over the Vietnam war.

"What stands out is the great philosophical debate that went on during that period, the Mansfields, the Fulbrights, the Cooper-Church amendment," Kimmitt said.

Others who gathered to drink and dine at the linen-covered tables, set up around the Russell Building's rotunda showed that Kimmitt and Hoffmann have followers in the Republican Party as well. Sens. Howard Baker (R-Tenn.), Richard Schweiker (R-Pa.), Nancy L. Kassebaum (R-Kan.), Strom Thurmond (R-S.C.) and Larry Pressler (R-S.D.) came to wish the two men well.

"Col. Kimmitt was a military man, and I knew him when he was with the Army liaison here [at the Senate]," Thurmond said. "He's a patient man, very well-organized. He's always performed his duties in an exemplary manner."

Other Democrats included prominent area party supporter Esther Coppersmith and Sen. Spark M. Matsunaga (D-Hawaii). Matsunaga and his wife presented Kimmitt and Hoffmann with blood-red carnation leis, which they brought in last night from their home state.

After Kimmitt and Hoffmann were presented with replicas of the pewter inkwells and the ballot box from the Senate floor, Hollings said both would receive personalized gifts from their colleagues. Kimmitt laughed with his wife, Eunice, when it was announced that he would get "a mobile telephone service." Hoffmann's wife, Joanne, widened her eyes in surprise when Hollings said their special remembrance would be "a giant-screen TV."

Earlier in the evening, a U.S. Army string ensemble had played "Somewhere Over the Rainbow." And, as of today, quite a number of those who've called the Senate home base will begin to drift off into an unknown land. No more receptions, no more roasts, no more petit fours in the Senate Caucus Room.

It showed on the faces at last night's party. Life after the Senate promises to be traumatic. Muskie said as much when he compared it to a now-common predicament.

"What we're all going to suffer," he said, "is a divorce. A divorce from public life."