Sen. Lloyd Bentsen (D), a Texas multimillionaire, put it bluntly: "If Moynihan becomes law, I couldn't stay in the Senate."

Bentsen was responding to an unexpected detour the Senate took last week while debating and voting on legislation to overhaul campaign finance. Seemingly out of nowhere, Sen. Daniel Patrick Moynihan (D-N.Y.) offered an amendment that would limit outside unearned income -- from dividends, interest, trust funds and capital gains -- of senators to 15 percent of their annual Senate salary, or roughly $15,000 a year. Moynihan's amendment was approved 51 to 49 with no debate and a short explanation from the author.

It would require, if it became law, at least one-third of the members of the Senate to give away thousands, perhaps millions, of dollars in income. Senators such as Edward M. Kennedy (D-Mass.), John C. Danforth (R-Mo.), John Heinz (R-Pa.), Herbert H. Kohl (D-Wis.) and John D. Rockefeller IV (D-W.Va.) would have to join Bentsen in resigning or restructuring their vast holdings of capital and property.

The Moynihan amendment was not viewed by many members as a serious vote. The legislation it would amend is considered to have little chance of becoming law since President Bush has threatened to veto any campaign finance bill that limits election spending. But its provides a glimpse into the resentments that surface when Congress deals with banning honoraria, voting itself a pay raise, or other matters that pertain to class and income.

Moynihan offered his amendment because, he said, he suddenly realized that a proposal by Sen. Christopher J. Dodd (D-Conn.) that banned honoraria also would limit earned income of members to 15 percent of their Senate salary.

"I was about to vote for the honoraria ban and saw this," Moynihan said in an interview. "There is something very important here," he said. "This is dividing the Senate between those who have large fortunes and those who have to live on limited earned income."

The Senate, he said, "is becoming an institution in which property is even more in evidence as a characteristic of its members."

Dodd, in introducing his amendment Aug. 1, made only passing mention to his intention to limit earned income. His press secretary, Julie Rosson, said Dodd had not hidden the fact that he intended to bring the Senate in line with the practice of the House, which voted late last year to ban honoraria and limit outside earned income, beginning in 1991. House rules, however, provide several exceptions to the earned income limitation, including royalties from writings, teaching income that is approved by the House ethics panel, pensions, and money derived from family businesses.

One critic of Moynihan's amendment said the New York legislator was "piqued" because Dodd did not indicate any exceptions and that Moynihan supplemented his income by writing and by teaching. Rosson said that Dodd expects the Senate ethics committee will write rules and exceptions similar to those in the House.

But Moynihan, a student of history, wrapped his amendment in a principle that goes back to the founding fathers, who made certain that Congress would include members of varying income levels who would be given a salary. "They wanted people of ordinary means who could serve in the national legislature," he said, "and not let it become a preserve of the propertied classes."

Moynihan said there was nothing wrong with having people in Congress from various walks of life, including those who must supplement their salaries through activities that are publicly disclosed. "The idea of writing into campaign laws {a} bias against earned income is reform gone wrong," he said.

When Moynihan proposed his amendment, shortly after the Senate had overwhelmingly approved the honoraria ban and the limitation on earned income, it was the luncheon hour and few members were on the floor. No one asked to speak either for or against his amendment, and when the vote was taken, the count was 50 for and 50 against. Moynihan appealed to senators to change their votes, and Frank H. Murkowski (R-Alaska) changed his from nay to yay.

One senator, who asked not to be identified, pointed out that Senate Republicans voted 31 to 13, more than 2 to 1, for the amendment. "The place where there was real class resentment was on the Republican side," the member pointed out. "They're assumed to be rich but aren't, and resent the Democratic millionaires."

Moynihan described the vote as one that "demystifies the class interest involved in all this reform behavior."

He pointed out that a majority of Democrats who are millionaires regularly voted against a pay raise for all members and for the ban on honoraria, none of which affect their standard of living. "That would leave many of us with not enough," Moynihan said, "and some with unlimited wealth."

There was a more cynical view voiced about the Moynihan vote from the other side of the Capitol. Rep. Bill Frenzel (R-Minn.) said some of the Republicans voted for a bill "they knew would not become law." A top staff aide to a Republican senator, who asked not to be identified, said that he knew "plenty of senators who would be surprised if Moynihan's amendment came back from the House-Senate conference."

Moynihan said that he'd be willing to drop his provision if the blanket limitation on earned income were also dropped. For several senators who voted for the measure, its enactment would mean serious changes.

For example, Sen. Nancy L. Kassebaum (R-Kan.) would do "whatever she had to -- including selling off assets before the January 1, 1991, effective date of the law," said a spokesman. For Kassebaum, who has hundreds of thousands of dollars in corporate stock and other major holdings, "it would mean a very dramatic change," the aide acknowledged.

Sen. Strom Thurmond (R-S.C.) reported receiving more than $40,000 last year in pensions. Under one version of the 15 percent earned or unearned limitation, he would apparently have to give most of that away. An aide said Thurmond "stood by his vote" and would do whatever necessary if the Moynihan provision became law.