Last November, the officers, professional staff, and rank-and-file employes of the largest transportation union in the AFL-CIO were urged -- as a "voluntary gesture" -- to contribute to its political action committee, the Railway Clerks Political League.

The appeal, on an RCPL letterhead, was signed by Fred J. Kroll, who is chairman of the PAC as well as international president of the Brotherhood of Railway, Airline and Steamship Clerks (BRAC).

The letter suggested -- "merely as a guide" -- a salary-related schedule for contributions, to be made preferably by payroll deduction. For a person earning up to $20,000 the proposed contribution was $100. For a top official earning at least $60,000 it was $600. Such schedules are legal.

Although the letter raised some eyebrows among the 106 clerical and staff employes at BRAC headquarters in Rockville, several say it was really the follow-up that angered them.

Of nine clerical workers who agreed to talk to a reporter if their names weren't used, seven, for example, said they had felt pressure to join the RCPL, mostly from unnamed staff supervisors who called them in -- individually or in groups -- or commented on their failure to sign up. None yielded to the alleged pressure, and none claimed to have been threatened or punished.

"I feel there is pressure," said one. She emphasized that she believes in the goals of the RCPL, which in the first five months of this year gave $185,530 to candidates, mostly incumbent House and Senate Democrats. But, she went on to say, "I resent the way they go about it."

Another worker, told the the federal election law forbids coercion, was asked if she felt she had been coerced. "Yes, I did," she replied. "It really angered me."

Such allegations were heatedly rejected by Kroll. He attributed them mainly to employees who, he asserted, had been reprimanded for poor performance and were trying to develop "a protective cover." He called them "liars."

"I know of no one who ever has been called in and encouraged, twisted, harassed or intimidated to contribute to our political league," he said in an interview at AFL-CIO headquarters. He offered to take a lie-detector test to verify that he neither knows of a noncontributer being called in by a supervisor nor has directed his staff to conduct follow-ups of that sort.

"I have nothing to hide," Kroll said. "As far as I'm concerned, we're totally clean on this."

In addition, Kroll produced records showing that numerous staff members and employes, including staff members and employes, including the seven who were interviewed, either hadn't joined the RCPL or gave less than the suggested amounts. One of the noncontributors was R. B. Douglas, who, Kroll said, he had "just promoted." Another was Gregg Donlon, son of BRAC's co-general counsel.

In May, six months after receiving the Kroll letter, holdout employes and staff members -- including several of the interviewed clerical workers -- were nudged by one of the union's international vice presidents, R. I. Kilroy.

In a letter on plain white paper without a letterhead, he reminded the holdouts of Kroll's appeal and, so that his point wouldn't be lost, enclosed a copy of it. He also enclosed a payroll deduction form.

As in the first round, according to some of the clerical workers, certain supervisors spoke to employes who remained resistant. Kroll denied this, although he allowed, toward the end of the interview, for the possibility that a supervisor had acted without his knowledge.

All of the interviewed employes declined to name their supervisors, saying they were afraid of providing clues to their own identities. One consented to an interview only on condition her words be paraphrased. Her complaint -- paraphrased here -- was, I dislike being nagged month after month.

The mere possibility of coerced contributions in federal elections is touchy because Congress made coercion a crime three-quarters of a century ago. t

The problem became acute for the Federal Election Commission in 1975, when it voted, 4 to 2, to let corporations spend their own funds to set up and administer PACs and to solicit certain employes.

The FEC majority, seeking to minimize "the appearance or perception of coercion," offered a guideline: "No superior sould solicit a subordinate."

The guideline made it into the Senate version of the 1976 amendments to the election law, only to be dumped in a House-Senate conference.

A possibility of involuntary contributions arose about 10 days after the Kilroy letter, when The Washington Post reported on the Sun Belt Good Government Committee, the PAC of Winn-Dixie Stores, the nation's fifth-largest grocery chain.

Four former administrative employes of the company disclosed that at individual salary-review meetings in June 1978, their boss, Atlanta division chief O. D. Carpenter, had followed up his god news -- a substantial bonus for each man -- with a solicitation for the PAC.

"I gave in the interests of my career," one of the men said. "You knew what the hell the point was," said another.

A BRAC employe who read the story phoned a reporter anonymously to complain that pressures of the kind cited by the former Winn-Dixie upper- and middle-level employes were as great, or greater, among the union's employes.

The caller, who refused to give out a phone number at which she could be reached, also said that just as in the Winn-Dixie PAC, information about the process of selecting recipients for RCPL contributions, and the amounts, was not volunteered.

Under the 1976 election-law amendments and FEC regulations, employes of unions and corporations are treated the same in some respects but not in others.

In corporations, clerical, production and other rank-and-file employes -- particularly if in a union's jurisdiction -- are out of bounds for company PAC solicitations. These must be limited to "stockholders and their families and . . . executive or administrative personnel and their families," FEC rules say.

At first glance, FEC rules for unions may seem to be stricter. They prohibit a labor organization or its PAC from soliciting contributions "from any person other than its members (the equivalent of stockholders) and their families."

For some unions, including BRAC, however, the prohibition is no obstacle to solicitation of employes -- whether administrative or clerical -- because the employes are members of the union that employs them.

Anyone who wants to work for BRAC, if not already a dues-paying member, must become one as a condition of employment. This is required by the union constitution, which says that employes "shall be members of the brotherhood at the time of their emloyment." Monthly dues for most members are $19.30.