The young advance man in the dark blue business suit leaned against the wall, alone and unnoticed, as the steel workers union hall emptied. The tension was finally gone from his face.

"Anyone who thinks the two-party system is dead should spend seven months advancing an independent presidential candidate," he sighed.

"Let's face it, coming to this place was pretty risky. For an advance person any uncertainty is anathema," he added. "Here we had a maverick local with an international union that had endorsed Carter. We were coming into a working-class area where we didn't have much organization support. It was a situation where we didn't have much control. We didn't know what to expect."

What Mark Rechtenwald, a 24-year-old advance man for John B. Anderson, ran into in this steamy-hot industrial city east of Los Angeles, was the kind of hardball politics the Anderson campaign has learned to expect from President Carter's reelection forces on the national level, but seldom sees so blatantly in the field.

Advance people are the traveling roadshow producers of American politics. They find the halls the candidates speak from, and try to fill them; they set up the microphones and the stands for the television cameras. They carry the bags, blow up the balloons, hire the bands, lay on some motorcades, paint the signs, and hang the banners. They rent hotel rooms, and run the Xerox machines, they cajole, beg, plead -- whatever is necessary.

The idea is to make the candidate look good, to make it seem that he is constantly surrounded by hullabaloo and throngs of wildly enthusiastic supporters.

Rechtenwald's problems with the Carter people began about noon Thursday, 28 hours before Anderson was scheduled to speak at steel workers' Local 2869 here. The appearance was an important, symbolic one for Anderson. The Illinois congressman has had trouble courting labor, and one of the most embarrassing moments in his campaign occurred last summer in Pittsburgh when he arrived for what was supposed to be a major breakfast with labor leaders, only to find the place almost empty.

First, local union leaders in Fontana received a telephone call from Jim Wood, field director of Carter's Los Angeles campaign office and associate director of the area's AFL-CIO Committee on Political Educuation.

Accounts conflict on exactly what Wood said. Anderson aides claim he asked, "What are you doing playing around with Anderson?" Wood told one reporter he called to say labor "is very serious about this election" and to ask if the local's invitation to the independent candidate was "official." He also threatened to distribute anti-Anderson literature during the congressman's speech.

Within the next few hours there was more pressure from Carter allies. "We got about a half-dozen calls," said union vice president Richard Flores."There were two calls from our district [steel-workers director], and one or two more from the Carter people."

The calls enraged local union leaders. The tactic backfired. "This local believes in the right of free speech for everyone.We invited all the candidates to come, and Anderson was the only one to accept," said union president Frank Anglin. "We don't buckle under pressure from anyone."

At 4:30 p.m., an inspector from the Fontana fire department arrived at the union hall. He found 13 violations and said the rally could not be held unless they were corrected.

The visit may have been coincidental to the phone calls. The local fire chief said he ordered the inspection after receiving a call from someone who identified himself as a Secret Service agent. The Secret Service routinely makes some calls.

But the inspection threw Rechtenwald's advance team into a crisis, and led to complaints of "Nixonesque dirty tricks" from local campaign workers.

"If they are trying to disrupt my appearance, it would be consistent with the pattern of harassment I have received all year from the Carter campaign," Anderson said Friday when asked about the pressures.

Rechtenwald was shaken at the news of the fire inspection. The campaign had spent days trying to build a crowd at the union hall.

Within the hour, Rechtenwald and his people made a series of emergency phone calls. A lawyer was sent to the fire department to get a written list of violations. Carpenters and electricians were called in. And the story of the alleged harassment was "leaked" to the Los Angeles Times.

The Anderson advance people hold a special place in the folk lore of 1980 politics. The standard was set last February in a Doonesbury comic strip. Michael Doonesbury, the star of the strip, met Anderson and was asked to become an advance man. "Congressman, what do you mean by advance?" he asked.

"In my case, it means you get out of the car first," Anderson replied.

The campaign has developed a cadre of advance teams since those early days. The members are bright, young, energetic, well meaning and incredibly clean cut. They have improved immeasurably over the months, moving Anderson and and entourage of about 60 reporters, Secret Service agents and staff members across the country and through Europe and the Midwest.

But by traditional standards, they are still amateurs, known affectionately as "the advance children" by the traveling press corps. And the campaign always has more bumps and false starts than the campaigns of the major party candidates.

The "advance children" have tried to play on the press' own weaknesses. Last summer when accounts began describing Anderson supporters as "the Chablis and Brie set," or the "quiche-eaters of the world," bottles of Chablis and layouts of Brie suddenly appeared on the press bus. Reporters loved it. When Anderson's chartered campaign plane set out on a trip last week, the first meal -- breakfast -- was, you guessed it, quiche.

But Anderson advance people are generally terrified of the press. They worry that a lost suitcase, a bad hotel, or a half-filled hall may result in an unfavorable news story, or even worse, a television news clip or syndicated column that reflects badly on the candidate.

Ask Rechtenwald why, he recites, almost verbatim, the passage from page 26 of "The Boys on the Bus," a book on the 1972 campaign. The passage talks about the slickness of the George McGovern advance and press operation. "When a reporter got to his room at night, his bag was there," author Timothy Crouse wrote, "And so pretty soon a reporter started saying to himself, half-consciously, 'If the press operation is this good, they must have a hell of a voter registration operation.'"

Rechtenwald is called a "lead advance man." He had three other salaried advance persons working with him during Anderson's two-day visit to the Los Angeles area last week, Beverly Hanson, Joan Trossman and Peter Bicks.

A short glimpse at their backgrounds offers an insight into the type of people who are the foot soldiers in American politics. When Anderson announced his candidacy more than a year ago, Bicks, for example, was a student at Pomona Collge in California. Trossman was a literary agent and part-time actress who had come to the Los Angeles area hoping to find a career in the movies. Hanson was the wife of a high school baseball coach and mother of four, who had decided "it was time to do something for myself."

She went to work for Sen. Edward M. Kennedy (D-Mass.), first as an office volunteer worker in New Hampshire and then as an advance person. After Kennedy's bid for the Democratic nomination failed, Hanson, bitten by the political bug, found a job with the Anderson campaign. When the campaign began, Rechtenwald was a young Harvard grad, growing bored with life in Washington and his work as a staff member on a congressional committee.

Last Friday was the advance team's day in the sun. They began working on it Monday. Trossman and Rechtenwald flew in from Hawaii where they had advanced a weekend trip for former Wisconsin governor Patrick J. Lucey, Anderson's vice presidential running mate. Hanson flew in from her home in Pennsylvania.

The day turned out to be one of the smoothest and most varied of the Anderson campaign. At the first event, the Compton High School band, a color guard, and at least one city councilman welcomed Anderson to a speech before a group of black Jaycees. A luncheon speech at the Disneyland Hotel to a group of Orange County businessmen and students was also a success. And the crowd at the steel workers hall in Fontana was so large it had to be moved from a basement meeting room into a larger auditorium.

Anderson gave a tough, combative speech, was presented a white hard hat, and drank beer with steelworkers. Rechtenwald was pleased. "I can't think of anything I would have done differently," he said.

And not a single suitcase was lost.