Rep. Tom Railsback (R-Ill.) finally took the plunge last month.

After serving 15 years in Congress, after delivering scores of speeches on how special-interest money can contaminate the political process, Railsback gave his first Washington, D.C., fund-raiser.

It was a highly profitable affair, as such parties typically are. Railsback estimates the fund-raiser brought in close to $50,000, much of it from corporate and trade association political action committees (PACs) that have an interest in the copyright laws his Judiciary subcommittee is writing.

Railsback's dipping into the PAC well holds the irony that he is author of a bill with Rep. David Obey (D-Wis.) that would put a limit of $70,000 on the total amount a congressional candidate could take from PACs in any two-year election cycle.

While the $33,075 Railsback has reported from PACs in 1981-82 is still well below that proposed ceiling, it represents a far heavier dependence on special-interest money than ever shown. In 1979-80, when his political competition was not as stiff as it is now, Railsback raised $14,981 from PACs.

But even as Railsback partakes more of PAC largess, some PACs have begun to punish him for his advocacy of the $70,000 limit. Railsback's primary opponent, Illinois state Sen. Kenneth G. McMillan, has received two $5,000 contributions from PACs opposed to the Obey-Railsback bill. The bill narrowly passed the House in 1979 but died in the Senate.

The two are the National Association of Realtors PAC of Washington, which gave more to congressional candidates in 1980 ($1.5 million) than any other of the 3,115 PACs, and the Public Service Research Council PAC of Alexandria, which opposes public-sector unions and argues that unions can use manpower to influence elections.

Railsback sees nothing hypocritical in aggressively seeking out PAC money this year. He notes that he limits himself to accepting $750 per campaign per PAC, well below the legal limit of $5,000.

"I've never been opposed to the concept of PACs," he says. "A lot of people have misunderstood that point. I simply favor limiting their contributions."

McMillan has raised $11,250 from PACs, about a third of Railback's total. That's par for the course. In 1980, incumbents received 61 percent of the PAC money contributed to congressional campaigns and challengers 26 percent; the rest went for vacant seats.

"If you're an incumbent, you get some perks--and one of them is knowing that you can always throw a Washington fund-raiser and get the lobbyists to belly up to the bar," said Dean Brown, McMillan's campaign manager.