House Judiciary Committee Chairman Peter W. Rodino Jr. (D-N.J.) says the proposal is unconstitutional. House Armed Services Committee Chairman Les Aspin (D-Wis.) has warned colleagues that it could severely damage the nation's defense.

Liberal Democrats have said they are convinced that it would savage social programs already hit hard by President Reagan's budget policies. But according to a key Republican House leader, some White House officials are "absolutely bonkers" because they think defense spending would suffer the most.

"The whole concept is nuts," said Joint Economic Committee Chairman David R. Obey (D-Wis.).

The "concept" is a deceptively simple plan to force a balanced federal budget by 1991. Sponsored by Sens. Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.), and known as the Gramm-Rudman proposal, the Senate-passed measure is under attack from all sides.

Nevertheless, the plan remains alive and apparently healthy because, in the words of a House Republican aide, it promises action on "the number-one political and fiscal issue in the country today" -- the skyrocketing federal budget deficit.

In this view, widely held on Capitol Hill, the idea of forcing Reagan and Congress to come to grips with the deficit has taken on a political life of its own.

Political momentum for passage also appears to be benefiting from many uncertainties about how the plan would work.

Because no one is sure if, or by how much, particular programs are likely to be cut under the bill's mandatory deficit-reduction provisions, Gramm-Rudman is a moving target. It pledges a steady deficit decrease without establishing specific and undoubtedly painful budget cuts that normally would generate strong opposition by lawmakers and interest groups.

In this atmosphere, said a House Democratic leader who asked not to be identified, "The House cannot kill this bill."

After a week of meetings that produced far more questions than answers, a House-Senate conference committee is expected to begin concentrating today on specific changes in the bill.

The committee faces a deadline at the end of the week because the proposal is part of legislation to raise the national debt ceiling to more than $2 trillion.

If the ceiling is not lifted by Friday, Treasury Department officials warned the committee last week, they will resort to the extraordinary step of temporarily disinvesting Social Security and other retirement trust funds, freeing enough additional credit to keep the federal government functioning until mid-November.

No one in Congress wants to tamper, even temporarily, with the trust funds. But there appears to be equally strong reluctance to raise the debt ceiling above $2 trillion without also moving toward mandatory deficit reduction.

As passed by the Senate, Gramm-Rudman would set mandatory annual targets for the federal deficit beginning at $180 billion this year and declining in $36 billion yearly increments until the budget is balanced in 1991.

If projections by the Office of Management and Budget and the Congressional Budget Office showed the deficit exceeding the target in any year, Congress and the president could approve budget cuts, tax increases or a combination of both to meet the target.

Without quick agreement on such a deficit-reduction program, the president would be required to impose across-the-board cuts on most federal programs except Social Security, which would be exempt.

Senate Democrats, a majority of whom voted for the bill, were clearly caught off guard by the proposal bearing the names of two junior Republican colleagues.

House Democratic leaders publicly decried the proposal while privately conceding difficulty in mustering a majority against a measure that promises to reduce and eventually eliminate the troublesome deficit. However, a Democratic strategy on the issue in the conference committee has begun to emerge.

House Majority Leader James C. Wright Jr. (D-Tex.) said Democrats have three priorities in shaping the final version:

*That budget cuts begin immediately, before the 1986 congressional elections.

*That Reagan's role in imposing budget cuts be purely "ministerial," leaving him no room to choose which programs to cut.

*That mandatory cuts triggered by the legislation fall evenly on all parts of the federal budget.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) has called Gramm-Rudman a "Republican incumbents protection bill" because the $180 billion deficit target for 1986 is higher than current deficit projections for the year and would not require Republicans, particularly 22 GOP Senate incumbents facing reelection, to vote immediately to cut popular programs.

Obey, chairman of one of four task forces established by House conferees to deal with the legislation, has proposed a Democratic alternative to fixed deficit targets set in Gramm-Rudman.

His proposal would base annual deficit targets on forecasts of economic growth each year, with projections of strong growth triggering a larger deficit reduction than forecasts of weak growth. Under this formula, the 3.4 percent growth rate predicted for 1986 would result in a deficit target of $159.6 billion, $20 billion less than Gramm-Rudman sets.

"We think it is a test of who is really serious about deficit reduction," House Majority Whip Thomas S. Foley (D-Wash.) said.

Democrats also will clearly press to limit, if not eliminate, any opportunity for the president to choose among programs in imposing mandated budget cuts.

In demanding that the ax fall evenly across the budget, Democrats have expressed particular concern that the defense budget bear a full share of any mandatory cuts.

As the conference committee began to learn last week, the simple promise of across-the-board cuts is not easily translated into reality.

According to analyses by the Senate and House Budget Committee staffs, a $25 billion mandatory cut triggered this year would result in defense budget reductions of $10.4 billion, $11.8 billion or $13.7 billion.

The different numbers result from different assumptions about what could be subject to what kinds of cuts -- questions that remain unanswered.

Looking over the figures, Rep. Sam M. Gibbons (D-Fla.) appeared to speak for many of his colleagues when he said, "There is a lot more in this thicket than we have been able to figure out so far."

Yesterday, administration and congressional leaders continued to debate the legislation. Interviewed on the ABC-TV program "This Week With David Brinkley," budget director James C. Miller III said Reagan would "absolutely" obey the requirement that defense programs be cut automatically if the bill becomes law.

Defense Secretary Caspar W. Weinberger has argued that Reagan could not be compelled to cut defense to meet a "rigid formula" for reducing the deficit. Weinberger has warned privately that Gramm-Rudman would seriously impair defense spending, although Reagan has promised to protect it from cuts.

Miller said yesterday that the impact on defense would not be as great as predicted by Aspin, but he acknowledged that the legislation "might be tough" on defense spending.

Miller said he does not expect Reagan to drop his support of the bill unless the House-Senate conference changes it to "hit" the defense budget more directly or impose tax increases.