DES MOINES, JAN. 28 -- Rep. Richard A. Gephardt (D-Mo.), the self-styled Mr. Outside of the Democratic presidential field, played his Mr. Inside card here today.

He did it with a supporting cast of 42 -- all members of the U.S. House of Representatives. They flew out together on chartered jet, donned their red "Gephardt for President" baseball caps, stood on the steps of the Iowa state capitol and gave group testimony to the fact that Gephardt, his antiestablishment rhetoric notwithstanding, is somone who can work with the system.

Or, as Majority Leader Rep. Thomas S. Foley (D-Wash.) put it, with an extravagance of praise familiar to congressional colleagues, "He is the most brilliant member of Congress in the modern period." He cited Gephardt's leadership on tax reform, Social Security and contra aid opposition.

As a campaign visual, it was the best choreographed event of the campaign so far. Foley and Majority Whip Rep. Tony Coelho (D-Calif.) led the House contingent up the steps of the capitol while Gephardt, his wife, his mother, Speaker of the Iowa House Don Avenson and a few dozen Iowa House and Senate members led a charge down the steps. They met in the middle and everybody hugged. The cameras whirred, the balloons flew and a few minutes later, the hard-times, blue-collar anthem to 1980s patriotism, Bruce Springsteen's "Born in the U.S.A.", blared over a loudspeaker.

The members span ideology and geography, but the music was a giveway: most are of Gephardt's generation. They are in their 40s,and were elected to Congress in the 1970s. Gephardt, as chairman of the House Democratic Caucus, the fourth-ranking leadership post, has emerged as their leader before he launched his bid for the presidency.

"I had a sense of history coming out on the plane," said Rep. Robert G. Torricelli (D-N.J.). "I wouldn't have thought it possible for 42 members to fly halfway across the country, on their own time, away from their districts and their families, to help out a colleague." He added that Gephardt's antiestablishment message struck a chord with others in the House because "being a progressive in Washington for the last seven years is the essence of being an outsider. This is the road back to influence."

The members did more than say nice words. After the capitol gathering, a separate car and driver whisked each one to 42 different counties, where they were scheduled to work senior citizens centers, union halls, students lounges, phone banks.

"I was in Ottumwa yesterday; it was my third time there in two months," said Rep. Martin Frost (D-Tex.), "and you can sense the momentum. This time, when I went to talk to the reporter for The Ottumwa Courier, he actually seemed happy to see me."

There is a yet another benefit Gephardt plans to extract. When the House votes next week on President Reagan's contra aid package, the floor leader of the opposition will be the man whose name was on all his colleagues' baseball caps here today.