Peter McCoy was given a Goodbye, Nancy Reagan and Hello, Malcolm Baldrige party at the Fairfax Hotel last night. McCoy, the first lady's staff director, is leaving the White House to become under secretary of commerce for travel and tourism. He said this is a very serious job.

"Tourism is either the first, second or third largest industry in 37 of the 50 states!" he exclaimed in the hotel bar, a wood-paneled room that was full of White House staffers and horse pictures. "It accounts for $10 billion a year in income for the United States! We have 20 million foreign visitors coming to this country a year! When a restaurant owner has an extra bus load of 50 people, that's 50 additional servings in a restaurant. That accounts for probably another dishwasher, or could be another cashier . . . ."

He was asked if this were the trickle-down approach to tourism.

"Trickle down?" he asked, then saw White House Cabinet secretary Craig Fuller. "It's trickled all over him," he said.

Many White House watchers were puzzled by McCoy's move, but his friends have said he wanted a job with more substance.

The party for 40 was given by John Coleman, owner of the Fairfax and one-time rising host-about-town. Lately, Coleman has spent much of his time in New York renovating a hotel, and so has surfaced on the scene less frequently. But last spring he did give a party for White House deputy chief of staff Michael Deaver and his wife, Carolyn. Coleman says he knows McCoy from "just around." To be more specific, they met during the Reagan campaign. McCoy and his wife, Kacey, have been to Coleman's New York apartment for dinner and so on.

Guests included retired Supreme Court justice Potter Stewart, Kennedy Center chairman Roger Stevens, Laughlin and Jennifer Phillips of the Phillips Collection, White House social secretary Muffie Brandon and presidential aides Joseph Canzeri and Morgan Mason. Mason was recently a subject of a magazine story that numbered him among Washington's worst dates.

"It hasn't dampened my life style in any way," he said. "I don't really think it's the kind of thing to hold a press conference about."

Dinner was consomme, watercress salad, lamb and mousse. The portions were elegantly small, as they often are at fancy parties, because most of the guests have to stay thin. McCoy, however, took several cookies for dessert. "These are all on my diet," he said. His Capitol Hill hearings for his new job are scheduled for this morning.

The toasts were short, although Deputy Secretary of Commerce Joseph Wright did go on about what a great job McCoy was getting. Immediately afterward, Craig Fuller crept over to McCoy's table and said: "But no car."

This is true. And when asked what he would miss most about his old job, McCoy -- who has never minded Washington parties -- replied:

"Invitations."