There was one thing that bothered Ronald Reagan, 74. Or as he put it to a black-tie crowd of U.S. senators last night:

"What I can't figure out is why anyone as young as Russell Long would retire."

Long, who turns 67 on Nov. 3, announced last winter that he will not seek reelection to his Senate seat from Louisiana next year. Last night he explained why, standing between Senate Majority Leader Bob Dole of Kansas and Texas Democrat Robert Strauss, his dinner hosts and also his Watergate neighbors.

"I looked at friends who made that decision to retire while they still had snap left in their garters," said Long, "and it seemed a pretty good idea."

Snappy garters or not, Sen. Edward Zorinsky (D-Neb.) said there were more senators at the Madison Hotel party than he usually sees on the floor of the Senate. Sen. Nancy Kassebaum (R-Kan.) said there were very few people "other than Russell Long" who could gather that large a group together.

Reagan's drop-by raised Long's familiar plea for bipartisanship to new expectations, at least for one evening.

"Take 'em to dinner and they're bipartisan," cracked Dole.

"No," he quickly amended it, just in case Long's feelings were hurt. "They're all Russell Long's good friends."

Certainly Ronald Reagan, fighting for passage on the Hill of his controversial tax reform package, could have passed for one as he stood there on the platform with Long and his wife Carolyn.

"For those who know Washington well, Russell Long is regarded as one of the most skillful legislators, compromisers, legislative strategists in history," Reagan said, mentioning a couple of his own predecessors who respected Long's talents.

"Famed for his capacity to anticipate truly critical moments in the legislative process, he was the one legislator who had his mind on the conference . . . when others were worrying about the floor debate," Reagan said.

Calling Long "the son of a political legend who has become a legend on his own," the president told a story about the Senate's number one tax expert.

"He used to ask, 'What is a loophole?' " said Reagan. "The answer: 'That's something that benefits the other guy. If it benefits you, it's tax reform.' "

Introducing cohost Dole, Strauss couldn't let Reagan's black business suit go unnoticed by the crowd. "Mr. President," Strauss teased, "I'm sorry your rich California friends didn't loan you the money to rent a tux."

Strauss, who was wearing an expensive-looking black-silk tuxedo, said he and his wife Helen, Transportation Secretary Elizabeth Dole, Ronald Reagan and the Longs were especially grateful to Dole for making the tribute authentically bipartisan.

"You're the only pure-blooded Republican in the whole crowd up here," Strauss told Dole.

Among the not-so-pure-blooded Democrats in the room was Michigan Sen. Donald Riegle Jr. (he switched his party affiliation in 1973), who wound up comparing noses with Reagan, not counting them. Riegle wore a bandage over the bridge of his nose where a basal cell carcinoma was taken out on Tuesday. It was the same type that the White House yesterday confirmed was removed from President Reagan's nose last week.

"My skin cancer was a little more complicated, and they had to fit my nose back together," Riegle said of the surgical procedure performed at the University of Michigan. "We chatted about it. He's got a strip of tape there but you'd hardly know it from a distance."

"Were you able to make bail after that fistfight?" joked Sen. Patrick Leahy (D-Vt.), spotting Riegle's swollen eyes and prominent bandage.

"Just what I need," Riegle laughed.

The crowd numbered 48 senators, three former senators and three Cabinet officers, with spouses. "We're all professional politicians," said Sen. Ernest Hollings (D-S.C.), "and Russell's the best. He's always known how to compromise, known the art of minority politics. You know, when the majority forgets you, the minority remembers."

Finances was a very big topic -- "Bob Dole gave me a lot of help when I was chairman of the Finance Committee," said Long. "I'd like to think I gave him a lot of help when he was chairman of the committee." But so were events surrounding last week's hijacking.

Sen. Alfonse D'Amato (R-N.Y.), chairman of the Senate Anti-Terrorist Caucus, said he understood the State Department was going to announce today a reward for the capture of or information leading to the capture of last summer's TWA hijackers.

And he told of a letter signed yesterday by 30 Senate colleagues asking Secretary of State George Shultz to do the same thing about the Palestine Liberation Front's Mohammed Abbas. "We can offer up to $500,000, and while it doesn't mean we'll catch him tomorrow, it's certainly something he'll have to worry about in the future. It's an incentive. It shows we're serious."

Long said the evening's turnout had impressed him. The "supreme compliment" was when Sen. John Danforth (R-Mo.) showed up at the very moment of the playoff between the Kansas City Royals and the Toronto Blue Jays.

Cautioned Strauss: "Maybe we better not report that Sen. Danforth was here. It might hurt him at home."