DON'T LOOK NOW, but the middle-of-the- roaders who have brought a budget to the Senate floor did pretty well last week. First, they faced down the Republican conservatives who have complained that the domestic spending in the proposed budget is too high. Then they brushed back the liberals who are inclined to regard that spending as too low.

Meanwhile, those who also argue that the defense spending in the budget is too low had a bad spell too. Defense Secretary Caspar Weinberger had to report that, despite all the talk of the end of the world when Congress cut his budget for this year, he had found an unexpected $2.2 billion in extra money rattling around. Lower-than-anticipated inflation and fuel costs were among the reasons. Critics pounced upon the news as proof the Pentagon doesn't need all it says.

The fiscal plan proposed by the Budget Committee would stray from President Reagan's priorities. To reduce the deficit while avoiding further deep cuts in most domestic programs, it would level off the defense build-up and modestly raise taxes. Majorities of both the committee's Democrats and Republicans supported this. But 24 Republicans said that they could not, on grounds that higher taxes and less for defense were the wrong ways to reduce the deficit and more should come from domestic programs.

Budget Chairman Pete Domenici and ranking committee Democrat Lawton Chiles thus began debate by offering an amendment to kill 44 domestic programs, mostly small, whose termination was proposed by the president. The list, not all made up of bad ideas, includes some entire agencies (the Small Business Administration, Economic Development Administration, Legal Services Corporation), urban development action grants, Amtrak and rural electric subsidies, direct lending by the Export-Import Bank, rural housing and juvenile justice programs and many more. Fourteen senators voted in favor of the amendment, 83 against, including 11 of the don't-give-an- inch-on-defense-but-cut-domestic-programs voters of a very short time before.

Next day came a liberal-led effort to save general revenue sharing, at a possible cost of more than $4 billion a year. It went down, 54 to 41. Efforts to add a much smaller amount for maternal, child and community health programs and to transfer funds from foreign aid to child immunization programs were also defeated. Those are hard votes in an election year.

In each of the last several years, the Senate has been the saving institution in the budget process, the one that finally had the courage to take the initiative when neither the president nor Democratic House would budge. The Senate is easy to poke fun at, but in every year what progress has been made toward a sane fiscal policy has begun there. Maybe, just maybe, those long-winded, overstuffed and occasionally underestimated creatures will pull it off again. Let's hope so.