By a vote of 56 to 43, with 12 Democrats defecting, the Senate adopted an amendment to Mr. Clinton's budget proposal that would prevent any additional defense cuts from financing domestic programs.
The vote puts the Senate in conflict with the House of Representatives, which has already approved a budget blueprint endorsed by the president that would take an extra $3 billion from the Pentagon over five years to help pay for Mr. Clinton's social investment programs.
The symbolic effect of the vote is greater, though, because it signals that Mr. Clinton cannot count on the sort of solid party-line support for his program that propelled it through the House last week.
"It's going to make the president's job a little more difficult," said Sen. Thomas A. Daschle of South Dakota, chairman of the Democratic Steering and Policy Committee. "It's going to make us have to go back and review some of the numbers."
Mr. Daschle said Senate Democratic leaders may try to undo the amendment before the Senate votes on the entire budget resolution late today or tomorrow.
The success of the amendment may be attributed largely to the enormous influence of its sponsor, Sam Nunn, the Georgia Democrat who heads the Armed Services Committee. Six of the 11 Democrats who voted with him serve on Mr. Nunn's committee.
Mr. Nunn, a force to be reckoned with under any circumstances, and Mr. Clinton got off to a bad start when the president tried to lift a ban on gays in the military without what the Georgia senator considered proper consultation.
A senior White House official played down the impact of Mr. Nunn's victory yesterday, saying the administration had put its energies into blocking a potentially more damaging Republican amendment that would have transferred $60 billion from domestic programs to the Pentagon. Only four Democrats, including Mr. Nunn, defected on that unsuccessful proposal.
White House officials and Senate Democratic leaders were also confident of beating back today another threatening Republican amendment that would strike from the budget Mr. Clinton's proposal to raise $32 billion by increasing the income taxes on Social Security benefits.
Mr. Clinton has proposed that single people earning at least $25,000 a year in outside income or married couples making $32,000 would pay tax on 85 percent of their Social Security benefits instead of the 50 percent now.
The White House apparently directed its most energetic lobbying yesterday at the Social Security amendment, and Senate Democrats were strongly urged to hold the party line on that vote at a party caucus.
The president also personally appealed to the senators at a news conference yesterday to approve his economic proposals intact.
"Each of these elements, reducing the deficit, asking the wealthy to pay their fair share, investing in the future and creating jobs, will work as a package," the president said. "And Congress should pass the package."
Mr. Clinton was also resisting attempts to trim by half his $16.3 billion economic stimulus package that the Senate is scheduled to take up before the end of the week.
"There are a block of people in the Senate, including some Democrats, who believe that the only thing that matters is to reduce the deficit," Mr. Clinton said. "Now, believe me, that's a big improvement over the past, but I just disagree with them.
"I believe that investing in the future matters too, and I believe if we don't change the spending patterns of the government and invest and put some of the American people back to work to create millions of jobs, that we're not going to have an economic recovery," he added.
The stimulus package, which includes money for summer youth jobs, highway and transportation projects, and unemployment insurance benefits, was approved intact yesterday morning by the Senate Appropriations Committee on a 19-to-10 party-line vote.
Mr. Clinton indicated yesterday he is more inclined to accept a compromise plan offered by Democratic Sens. John B. Breaux of Louisiana and David L. Boren of Oklahoma that would simply delay some of the spending until the full economic plan is passed later this year.
"But the problem with phasing it in is, if you delay the investment, you also delay the impact of the investment, which means you put off the effective date of the jobs being created," Mr. Clinton said.
But as a practical matter, Clinton aides say about half the spending in his $16.3 billion package could not be undertaken until fall anyway.