Rep. Tom Railsback (R-Ill.) said yesterday he and two congressmen colleagues "made a big mistake" when they shared a Florida cottage during a golfing vacation last year with a woman lobbyist who later posed nude for Playboy magazine.

Railsback said he and Rep. Thomas B. Evans Jr. (R-Del.) shared a house with six other men and Paula Clifton Parkinson, 30, who was featured in a Playboy article called "The Women of Washington."

Parkinson said she was lobbying against legislation that would have extended federal control over crop insurance, but Railsback said, "Frankly, Paula wasn't even sure what side of the issue she was on. Lobbying isn't her strong suit."

Evans, who went into seclusion after a news account of the trip appeared in the Friday edition of the Washington (Del.) News Journal, issued a statement saying, "I deny any of the vicious implications. . . . I have never let any influence other than the best interests of the citizens of the state of Delaware and my country affect my vote."

The third member of Congress who stayed at the house was Dan Quayle of Indiana, who was then in the House but since has been elected to the Senate. Quayle said yesterday that he spent two nights at the house and "couldn't vouch" than any women were there.

I wouldn't know her [Parkinson] if she walked in my office," Quayle said. "I went down there to play golf, and that's what I did. I was in bed both nights by 10:30." Quayle said he shared a room with William Hecht, a lobbyist for the Tobacco Institute. "If you want to make anything of the trip, I guess you might try to make something homesexual out of it," Quayle joked.

Parkinson could not be reached for comment yesterday. But it was learned that she has said she had affairs with half a dozen members of Congress and has been attempting to sell information about sexual escapades to both Playboy and Penthouse magazines. Rudy Maza, who writes a gossip column for The Washington Post Magazine, said yesterday he had spoken to an editor at Penthouse to see if the magazine was interested in buying Parkinson's story.

Railsback, interviewed from his home in Illinois, said "when we got to the house and saw that Paula was there, we should have either asked her to move out, or we should have moved out." He said he had known Parkinson previously but that "she didn't proposition me. I didn't touch her."

As to the sleeping arrangements during the week, Railsback said "we all had to double up." He said his roommate was a Delaware obstetrician, Dr. John Garrett.

Quayle and Railsback said the trip was an annual golf outing on which they were joined by Rep. Marty Russo (D-Ill.), who spent that week with his brother in nearby Palm Springs.

The congressmen were supposed to have had a place to stay free of charge with a Delaware golfer, Wally Sneza, according to Quayle, but "when Marty Russo picked me up at the airport, he said Wally's place was already occupied. I had my choice of staying with Russo or with Evans and Railsback, and I chose their place because it was closer to the golf course.

The house the men rented was in a golfing community south of Palm Springs called Atlantis. Another member of the party, according to Quayle, was a friend of Evans from New York named Fred Tuck.

Railsback said the men played golf every day, and that one night, all of them went out to dinner with Parkinson. "Everyone had a good time, but that's all there was to it," Railsback said.

Quayle said he didn't remember the name Parkinson, but that he "may have heard" her maiden name of Paula Clifton. Quayle said that Parkinson called his campaign office last October, at the height of his tight race against incumbent. Sen. Birch Bayh, and said "something about Jack Anderson was going to print that she had an affair with me," Quayle said. The item never was printed.

"Now this, five months later," said Quayle. "I don't know the girl. If she was in Florida, if I met her for a fleeting momemt, fine."

Railsback said Parkinson was not at the house the first day of the vacation, but "there she was" when they returned from golfing on Jan. 4 or 5. He said he didn't know who had invited her, or how she had gotten to Florida.

"Not moving out was my first mistake," said Railsback. "Not telling my wife about it when I got home was the second."

Railsback said he told his wife, who is recovering from surgery, this week. "I feel terrible about this," Railsback said, "but I swear -- and I'd take a lie detector test, not one single thing happened between us."

Railsback said he didn't see "anyone in bed with her" during the week. "I know you think I'm trying to protect my friend Tom Evans, but it's true. I didn't know anything about bedtime arrangements."

As to Parkinson's lobbying effort, Quayle said: "It wouldn't make any sense to lobby a midwestern Republican to vote against crop insurance. We were all against it in the first place.

Evans, who still holds part interest in an insurance firm in Washington, said the legislation "was opposed by both the American Farm Bureau and the Delaware Farm Bureau. It poses the serious threat of nationalizing the crop insurance industry . . . contrary to the strong public sentiment against excessive federal intervention and unnecessary federal spending."

Over the objections of Evans, Railsback, Quayle and others, however, the proposal became law last September, after passing in the House by a vote of 235-150.