It sounds like a scene out of a bad political novel. A pipe-smoking lobbyist visits the office of a young congressman and plays a tape recording of a damaging radio ad. Then, the lobbyist threatens to flood the airways of the congressman's home district with the ad unless he agrees to support a bill, which neither lobbyist nor congressman has seen.

The congressman is outraged. The ad isn't true, he declares. It's almost true, the lobbyist replies. Besides, politics is politics.

The congressman, steaming with anger, begins a lecture on honesty and decency. Then he pulls out a Bible and begins reading from Proverbs 26:4: "Answer not a fool according to his folly, lest you be like him yourself." And from Proverbs 26:6: "He who sends a message by the hand of a fool cuts off his own feet and drinks violence."

The story is true. It happened Thursday. The lobbyist was William R. Worthen, legislative director of the National Tax Limitation Committee, which is working to support President Reagan's budget and tax cuts. The congressman was Rep. Bill Nelson (D-Fla.).

Worthen's group has decided to pressure nine conservative Democrats, including Nelson, to support the latest round of budget cuts before the House. All nine have supported the cuts in the past.

If the congressman don't agree, Worthen said his committee will mount radio ad campaigns in their home districts next week. It has budgeted almost $9,000 for this purpose.

The ads, Worthen said, were "the brainchild" of David Keene, political director of George Bush's unsuccessful presidential bid and a consultant to the tax cut group.

The ads say House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) "has devised a scheme that makes a mockery out of the president's budget. O'Neill is saying, 'Tell the folks back home you voted for Reagan last month. That will satisfy them. But vote the party line today.' You see, Tip O'Neill really believes he can fool most of the people most of the time."

The tactic may backfire. "This is intimidation. It is blackmail. This is reverse bribery," Nelson said late yesterday. "If they play these ads, I definitely won't vote with them, on principle."

The second-term Democrat, whose district includes Orlando, was particularly upset that the bill Worthen asked him to support didn't even exist when the lobbyist visited him. The bill, called Gramm-Latta II, was made public yesterday.

Worthen said Nelson was trying to "make a tempest out of a teapot. I don't know how we could have been more honest," he said. "I just wanted the congressmen to know what we were up to."

The lobbyist clearly was taken back by Nelson's Bible-reading. "It's always good to hear a good speaker read the Bible," Worthen said. "But there are more pleasant experiences that having a congressman standing above you quoting from the Bible."

When Worthen came to visit, Rep. Daniel A. Mica (D-Fla.) turned on a tape recorder and grilled him. "The lobbyist hadn't read the bill. He didn't know what was in it, and here he wanted the congressman to agree to support it," said Rita Highbaugh, Mica's press secretary.

Other conservative Democrats targeted for the ads are Rep. Donald J. Albosta (Mich.), Rep. Ronnie G. Flippo (Ala.), Rep. William V. Chappell Jr. (Fla.), Rep. Bo Ginn (Ga.), Rep. Jack Hightower (Tex.), Rep. Earl Hutto (fla.) and Rep. L. H. Fountain (N.C.).