It was Howard Baker's first day on the job, and as he noted at a party in his honor last night, the title "Senate majority leader" took a little getting used to. He wasn't alone; his predecessor from West Virginia, Robert Byrd, also had a new title to get used to: Senate minority leader.

"Byrd and I stumbled over it about three times today," Baker said between the fish course called "supreme de fruits de mer" with sauce Louis and the meat course of escalopes de veau, "but deep down we both understood."

Baker's big day, which reached its climax at a posh black-tie dinner in his honor at the Corcoran Gallery of Art, found "the boss" himself -- President-elect Ronald Reagan -- flying in from Mexico to add a grateful pat on the back to all the others being heaped upon the Tennessean.

"Howard's not only a great leader," said Reagan, looking a little weary after his whirlwind one-day tour south of the border to meet Mexico's President Jose Lopex Portillo, "but in the campaign he did as much as anybody could, a large part of the reason why he is now majority leader.

"Now that I'm being provided public housing in this city," Reagan continued, "I'm looking forward to a closer relationship."

Heady stuff for a Republican leading the pack in out of the cold for the first time in 26 years, and Baker didn't bother to conceal his pleasure.

Or as Baker's pilot, Lonnie Strunk, put it a little later during toasts: "This is one big deal of a dinner you got going here. I used to wash his car back in Tennessee and I knew that one day I'd be at a big fancy dinner but I never dreamed he'd be here, too."

It got the biggest laugh of the evening.

The "big deal of a dinner" celebrating the opening of the 97th Congress and Howard Baker's ascension as majority leader, was the handiwork of the publishing powerhouse Times Mirror Co. whose interests include The Los Angeles Times, Newsday, The Denver Post and The Dallas Times Herald. Otis Chandler, chairman and editor-in-chief, played host, sharing receiving line duties with Robert Erburu, president and chief executive officer, and Franklin D. Murphy, chairman of the executive committee.

The youthful-looking Chandler was ebullient over his catch of the evening, 400 of Washington's leading legislators, columnists, network heavies, culture mavens and socialites. He and his associates gave the party, he said, because Baker was an old friend and besides, he joked, "we have a party every Monday night in some city around the country."

Chandler ordered dinner delayed so Reagan could be brought by motorcade from Andrews Air Force Base. When the president-elect saw his host and all the others in the ubiquitous black tie that becomes Washington's uniform after dark, the former California governor apologized. "Not only for not being in black tie," he said, "but I'm in a brown suit."

And then, in what seems to be a favorite source of Reagan jokes, he drew from his show-biz repertoire to tell about an entertainer who thought he had knocked everybody dead while he was out on stage. Later, backstage, his agent had a different perspective. "Never wear a brown suit again," the agent told him.

Reagan described his meeting with Mexico's president as "a very successful and wonderful meeting establishing a basis for the kind of relationship neighbors as close as we are should have."

He sat down briefly -- NBC's Roger Mudd politely relinquished the seat at Baker's table -- while Baker responded with what he called "a final word." He was "absolutely delighted to be in the majority," Baker wryly told Reagan and the crowd of 400, "but there is also a certain responsibility -- we've done nothing but scratch and fight to try to get your nominees confirmed."

But last night, at any rate, there was no scratching and no fighting but lots of kind words.

"I said, 'Just for Jody to survive for four years was something,'" said Nan Powell of husband Jody's White House tenure as President Carter's press secretary. And Howard Baker, walking with the Powells, some of the New Outsiders, said, "Listen, to survive is everything."

Said another New Outsider, former Carter campaign chairman Robert Strauss, spearing a pasta hors d'oeuvre, "I'm glad Baker put me on the list tonight."

And from yet another, former White House counsel Lloyd Cutler, who left his post a month ago: "I follow the 'last in, first out' rule."

And then, in what seems to be a favorite source of Reagan jokes, he drew from his show-biz repertoire to tell about an entertainer who thought he had knocked everybody dead while he was out on stage. Later, backstage, his agent had a different perspective. "Never wear a brown suit again," the agent told him.

Reagan described his meeting with Mexico's president as "a very successful and wonderful meeting establishing a basis for the kind of relationship neighbors as close as we are should have."

He sat down briefly -- NBC's Roger Mudd politely relinquished his seat at Baker's table -- while Baker responded with what he called "a final word." He was "absolutely delighted to be in the majority," Baker wryly told Reagan and the crowd of 400, "but there is also a certain responsibility -- we've done nothing but scratch and fight to try to get your nominees confirmed."

But last night, at any rate, there was no scratching and fighting but lots of kind words. To those accustomed to the changing of the guard and the supporting casts, breaking the evening's guest list into categories was part of the quadrennial game:

The New Outsiders -- "I said, 'Just for Jody to survive for four years was something,'" said Nan Powell of husband Jody's White House tenure as President Carter's press secretary. And Howard Baker, walking with the Powells replied, "Listen, to survive is everything."

Robert Strauss, former campaign chairman to President Carter, looking about as bipartisan as a Texan can look, speared an hors d'oeuvre from a basket of pasta and said: "I'm glad Baker put me on the list tonight."

From former Carter counsel Lloyd Cutler, long a lawyer for the Los Angeles Times who left his White House post a month ago: "I follow the 'last in, first out' rule."

The New Insiders -- Transportation Secretary-designate Drew Lewis and his wife were making their Washington debuts. "I don't know if you'd call it a debut," said Lewis uncertainly. "It's our first party," said his wife, firmly. She returns to Pennsylvania today to be sworn into the state legislature.

Nevada Sen. Paul Laxalt, commenting on his role in the new Senate Republican leadership as announced by Baker earlier: "I was sort of floating around with no place to go. I really appreciated it." Laxalt said he had decided about a month ago not to move into an office at the White House because "it raised some legal problems -- you know, the separation of executive and legislative branches." But they'll still let him in the White House, won't they? "Oh, occasionally," he grinned.

Media Moguls -- "Howard Baker is the most natural politician I know, the best adjusted, someone who really knows who he is," said syndicated columnist Joseph Kraft, deciding that was why Baker didn't win the nomination for president but also why he's got "nine lives."

Syndicated columnist Art Buchwald, inspired by the California dress code, sported a brand new jacquard dinner jacket by Adolfo. "After I heard Nancy Reagan went for Adolfo, I said, 'you gotta get with it.'"

Also in the crush, sans Adolfos, were columnist Carl Rowan. The Washington Post's David Broder, The New York Times' William Safire, ABC's Barbara Walters and Frank Reynolds.

Legislators from both sides of the political aisle -- South Dakota's Sen. Larry Pressler, who made history earlier in the day by being the first Republican not a member of the leadership to preside over the Senate in 26 years. "It was only for 15 minutes and quite by accident, but I felt really powerful."

Maryland's Sen. Paul Sarbanes was one of several Democrats claiming their new secondary status hadn't sunk in yet. The sight of three wine glasses at each plate on the tables covered in purple moire moved him to speculate, though, "That's to help it sink in, eh?"

Louisiana's Russell Long took the long view. "A pleasant day," he said benignly. "Being from the South, I've been in the minority in many respects anyway."