One of the season's most unusual social events occurred last night at the Federal City Club, when some 90 leading members of the Washington Power Elite sat down to dinner with some 50 tourists from California.

When they began chatting to relieve the banality of the menu (chicken, pilaf, boiled carrots and a weak South African Riesling for table wine), the two segments of the party found that they had one thing in common: Everybody there liked George S. Mitrovich.

Mitrovich, a native of San Diego, spent five years in Washington working as an aide to various members of Congress and running an exclusive informal luncheon club. He is now a political consultant in his home town, and is generally acknowledged by the distinguished guests (a very tough-minded group) to be one of the most persuasive men in the United States.

As founder, president and work-horse-in-chief of the City Club of San Diego, he has in the last three years brought to that city some of the nation's most widely sought (and expensive) speakers on a budget of about $1,000 an event.

He gave last night's dinner as a sort of payment for those speakers (who usually charge up to $5,000 per talk) who had flown out to San Diego and addressed the City Club without charge.

A few of the invited guests were unable to make it (Jimmy Carter, Gloria Steinem, Eugene McCarthy) but a substantial representation was present from Congress - including Sens. Joseph Biden (D-Del.), William Cohen (R-Maine), Alan Cranston (D-Calif.), Dennis DeConcini (D-Ariz.), Mark Hatfield (R-Ore.) and Charles McC. Mathias Jr. (R-Mid.)-and from the media.

A few of the media people were singled out for the spotlight by master of ceremonies Mark Shields, who introduced CBS' Roger Mudd as "the John Travolta of the League of Women Voters" and accused columnist Robert Novak of a sort of fraud: "There isn't any Evans; he knew that in order to be put in the San Diego Union, he needed a WASP on the letterhead."

Mercifully spared Shield's spotlight were such guests as Ben Bradlee, David Halberstam, George Plimpton, Sally Quinn, Richard Reeves, Dick Tuck, Myra MacPherson and Sander Vanocur.

Also present were approximately 50 members of the City Club of San Diego, who are spending a week in Washington at a cost about $500 per person-the price including last night's dinner with a certified national political or media personality at each table.

"You know, San Diego is sort of at the end of the line in transportation and communications," said one of the California guests, Danah Fayman, president of the Museum of Contemporary Art in La Jolla. "George keeps us in touch with the world-those who want to be kept in touch."

Mark Shields put it a bit differently before going up to serve as master of ceremonies: "George has this program of bringing Americans to California."

Many of those who have spoken at the City Club call Mitrovich a gentle arm-twister, and he happily admits it. "Dick Reeves has come out four times and doesn't even charge expenses," he said. "He said my secret lies in two things: persistence and the fact that I call him on California time. That means I'm constantly waking him at 2 a.m., when his resistance is low."

He says that the City Club reminds him of his old days in Washington, and "without it, I would not be able to continue living in San Diego."

The club has about 500 members, about half of whom are women. Couple pay the same dues ( $100 per year) as individuals, so there are only 300 paying memberships, and Mitrovich believes it has to grow because "you can't sustain yourself forever on the kindness of your friends." CAPTION: Picture, George Mitrovich, Steve Martindale and James Abourezk at last night's dinner, by Harry Naltchayan-The Washington Post