Seattle Mayor Charles Royer was still beaming after working the ninth cocktail reception of the evening, but his telegenic smile slowly faded when he returned to his hotel room.

It was the end of a triumphant day for the dark-haired Democrat--a day in which he played host to President Reagan, was courted by the national press and prepared to assume the presidency of the National League of Cities.

But Royer, 43, soon discovered that Seattle newspapers were carping at him for taking "an entourage" of 12 persons to the league convention in Los Angeles last month and questioning whether his frequent absences were hurting his ability to manage the city.

"We get a third of our budget from the federal government," Royer snapped, throwing one of the offending newspapers on the coffee table. "How am I supposed to lobby for that? By sending a Mailgram?"

Royer is a political enigma whose rising national prestige is matched by considerable criticism at home. A nightly television commentator for Seattle's KING-TV before he ran for mayor five years ago, he is surprisingly sensitive to negative media coverage. A liberal with no previous management experience, he shocked followers by naming his younger brother as deputy mayor.

"Charlie has marvelous talent for making an error and coming out smelling like a rose," Seattle City Council President Jeanette Williams said. "He is an excellent spokesman for national issues, he's articulate and has a wonderful sense of humor. But that has very little to do with running the city."

Still, Royer managed to get himself reelected last year amid a deep recession, even after raising taxes and slashing city services. His years behind the camera may have prepared him for a larger role at a time when federal aid to cities is shrinking rapidly.

With his knack for the telling phrase, Royer calls Reagan's urban policy "a blueprint for surrendering America's cities." He served up a local austerity budget that he termed "as bland and spare as macaroni and cheese." And, when the league wanted to dramatize the need for federal revenue sharing, Royer tagged the aid "mother's milk for cities."

Royer sees federal programs as a vital investment that has helped his city weather the hard times that followed massive layoffs at Boeing Co. in the early 1970s.

Three years ago he was able to obtain $150 million in federal aid to repair the West Seattle Bridge after it was damaged by a merchant ship. That aid would not be available today, Royer says, and Seattle could not afford to fix the bridge on its own.

"What is a national responsibility?" he asks. "Is it a national responsibility for Puget Sound waters to remain clean? Or should the city of Seattle have to triple sewer fees to keep them clean?"

Despite Seattle's limited home-rule powers, Royer has struggled to make the city more self-reliant. In the last two years he has raised the sales tax, slapped a 40 percent surcharge on local business, cut police and fire service, laid off 5 percent of the work force, closed the consumer protection office, floated a bond issue to build 1,000 low-income apartments, taken over a Public Health Service hospital and imposed fees on everything from parks to building inspections.

Some cutbacks proved impossible. Royer likes to recall how he tried to close the city-run Bathhouse Theater, only to watch theater lovers march downtown and persuade the council to cut $80,000 from the road-paving budget instead. "Very few mayors go down in history as great maintainers," he joked.

The views of others who take a harsher view of Royer's management skills are reflected in the words of one Seattle official: "Charlie has not accomplished much for the city. He likes to take credit for things other people have accomplished. And some of the department heads he's appointed have been absolute disasters."

The aggressive mayor has refused to abandon close aides who got into political hot water, even after a flap over a city contract awarded to his brother's wife. And while he criticized his predecessor for junketing, he cringes when reporters mention his recent travel schedule, which included Honolulu, Miami, Vancouver, Georgia and five trips to Washington, D.C.

"Charlie is extremely sensitive," a local politician said. "He takes everything as a personal assault on him." When one officeholder was quoted as mildly criticizing him, Royer would not talk to her for a week.

Royer acknowledges these traits with disarming candor. "I want to do well so badly," he told The Seattle Times. "In journalism, I saw people who didn't do well and, damn it, I'm not one of those."

But his idealistic streak is tempered by a strong dose of caution. Royer toned down his liberal rhetoric at the League of Cities convention, where many of the delegates are conservative and from small towns, and he avoided criticizing Reagan. Instead, he talked about "a limited spectrum of what's possible."

Royer places no such limitations on his career. He makes little effort to discourage rumors that he is ready to run for governor or angling for a future Cabinet post under a Democratic administration.

"If you do something well," he said, "you ultimately want to be in charge of it."