A hearing on senatorial ethics became a verbal brawl yesterday when two representatives of Common Cause accused the Senate of failing "to take ethics seriously."

The charge was made by Common Cause executives David Cohen, president, and Fred Wertheimer, vice president for operations, in a statement before the Senate Special Committee on Official Conduct.

The committee has until March 1 to draft an ethics code proposal for presentation to the Senate.

"The Senate's need to create a special committee to draft a new code of official conduct is tacit admission that in the past it has failed to take ethics seriously," Cohen and Wertheimer delared in their prepared statement, which Cohen read aloud.

"It is a confession that the Senate as an institution has been ignoring very serious problems," Cohen continued.

"Er, um, hold it there. Hold it there," interrupted a redfaced Sen. Gaylord Nelson (DWis.), committee chairman.

"When you said that, you indicted everyone in the Senate as having a low standard of ethics," Nelson said. "I reject that as being demagogic . . . I think it's not only demagogic, it's McCarthyistic to indict every single senator," Nelson scolded.

He tried to rein in his temper, and adopted the demeanor of a scolding professor.

"I think semantics are very important," Nelson said. "You have a responsibility to be careful what you say, as do members of Congress. When we're irresponsible, we got public criticism. And I think you should get public criticism when you're irresponsible."

Still redfaced. Nelson pointed out that he and many of his colleagues have been strong supporters of a meaningful ethics code. Certainly that is proof that neither he nor his colleagues have ignored the matter, he said.

Cohen said he and Wertheimer meant personal. But Cohen said, "The fact that the public believes the cleaning up of corruption in government should be the top priority of the new Congress is a terrible indictment of the state of congressional ethics."

Cohen then called for full financial disclosure and an "understandable code of ethics" that is enforceable.

And then, in a rising voice, he said: "My final plea to you is to say that we want the Senate and the House to be respected institutions . . . We want to have as much respect for them as we do the flag. Don't misread the public's views. Don't let us down."

The statement obviously upset Nelson and other members of the committee, who had been conducting the hearings for two days in relative tranquility.

Most of the witnesses who appeared before the committee representatives of public interest groups. None of them was as openly critical of Senate's performance as the two Common Cause representatives.

The committee grilled the Common Cause witnesses for almost two hours. Sen. John Melcher (DMont.), a former House member, became indignant.

He implied that Common Cause, with its self-avowed goal of protecting the public interest, was no different from any other group seeking to influence Congress.

"You are a lobbyist," Melcher told Cohen, pronouncing the word "lobbyist" in a somewhat sinister tone.

Later, Melcher added: "I disagree with the idea of David [Cohen] coming up here and waving the flag at us. We're sensitive, David, to the idea of the flag going by, too."