Sen. Warren B. Rudman (R-N.H.) said of the automatic deficit reduction measure that bears his name and passed the Senate for a second time yesterday: "It's a bad idea whose time has come."

Rep. Leon E. Panetta (D-Calif.), who helped push a Democratic alternative through the House last week, said if Rudman's handiwork becomes law, the U.S. government will be "leaping into a black hole, and we don't know where we're going to land."

Rep. Willis D. Gradison Jr. (R-Ohio) said he thinks the bill's automatic spending cuts will be "arbitrary, mindless and irrational." Why is he in favor of them? "When logic fails, you try illogic."

"It's an amazing chemistry that's going on here," Sen. Alan K. Simpson (R-Wyo.) said. "We're like Igor, the mad scientist, in his laboratory." He added: "The American people are offended by this Mickey Mouse brinksmanship. They think it's absurd. But I'm going home and telling them that this is the only way I could get it done."

Rep. Marvin Leath (D-Tex.) said the whole idea of automatic cuts is "a disastrous way to govern." He's for it, though.

The Gramm-Rudman-Hollings deficit reduction bill has achieved a distinction rare in the annals of legislation: It has managed to unite an ideologically diverse coalition of supporters around the proposition that it's a thoroughly bad bill. For six weeks, its proponents have been routinely denouncing it as an abdication of congressional authority, a leap into the unknown, a blunt instrument aimed at a complex problem, a mousetrap likely to bite the tail of an administration that this week is having serious second thoughts about supporting it, and a Rube Goldbergesque, crisis-creating contraption that is probably unconstitutional to begin with.

But over those same six weeks, the concept has retained strong support, in part because it is tied to legislation that must be approved, to increase the U.S. debt ceiling by mid-November, and in part because all of the legislation's substantive weaknesses have turned out to be its tactical strengths.

It has landed in Congress at a moment of abject frustration over the inability to make a dent in the deficit, and it is ingeniously crafted to play on this low institutional self-esteem as well as on the mounting fear that the deficit issue may finally have a political sting across America.

The very uncertainty about whether the automatic spending cuts would ever be invoked -- and there are strong opinions in Congress on both sides -- turns out to be an engine for passage, too.

It has lent just enough of a patina of unreality to the whole debate to convince some lawmakers that they are merely casting a symbolic vote. Several said they take at face value Secretary of Defense Caspar W. Weinberger's threat that if the automatic cuts were to take too big a slice out of the Defense Department budget -- and nearly everyone agrees that defense is more exposed than any other department -- President Reagan would simply ignore the law or insist that Congress or the courts overturn it, perhaps by declaring he has superceding reponsibilities as commander in chief.

"Obviously the weakness is it can be postponed, modified, whatever happens," said Senate Majority Leader Robert J. Dole (R-Kan.), a tepid supporter. "If we get up to the crunch, we can always change it."

But many other members -- perhaps a majority -- are convinced that if a bill does emerge from House-Senate conferences, and if Reagan signs it, an approval that is no longer certain, then the automatic spending cuts really would be invoked, probably as early as December or January.

The jockeying this past month has been over where the "automatic" ax would fall. In that sense, the bill has been a moving target. As initially presented, it was a strictly procedural solution to the deficit dilemma, ideologically neutral in its insistence that "across-the-board" cuts be applied whenever deficit reduction targets over the next five fiscal years were not met.

But true neutrality is proving a phantom goal. Immediately, Social Security was exempted from the cuts in order to build consensus for the bill. Legal complications over contractual obligations surfaced suggesting some programs would be more exposed than others.

Once cracks developed in the across-the-board approach, House Democrats had an opening to turn a "procedural bill" into a forum for replaying the guns versus butter argument that has been central between the parties for a generation. Democrats say they now believe that the public is more willing to cut defense than domestic programs.

The House alternative exempts programs for low-income people and veterans' pensions from the automatic ax, thus leaving the Defense Department to bear up to two-thirds of the load. The Senate bill spreads the pain more evenly, and leaves the Pentagon less exposed, although still vulnerable.

Though differences between the House and Senate bills are considerable, Democratic leaders said there is room for compromise, and they expect one to emerge from the conference committee. That is because, after first being caught off guard by the Republican thrust, they now believe their party is poised to benefit from the process of deficit reduction, whatever its final form. Once the cuts begin to fall, Democrats said they believe, the spell of pain-free Reaganomics will be broken.

"This [deficit cutting] is what the public wants, but they have to understand what it means," said Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee. "Once the elderly understand it means cuts in Medicare, they're going to come home to the Democratic Party ."

Depending on which version passes, if one does, and whose estimates one believes, the bill could mean that the Defense Department would have to fire 674,000 military personnel (one-third of the armed forces) by fiscal 1990, mothball 16 of the 24 vessels under construction, eliminate effective Pentagon programs along with ineffective ones, and give ground on the MX missile, the B1 bomber and the Strategic Defense Initiative. Or it could mean that 2 million low-income children lose access to Medicaid benefits, Headstart centers close all over the country, 600,000 poor pregrant women lose their nutritional supplements, and millions of elderly see their Medicare benefits reduced.

But these Draconian scenarios only serve to highlight one final -- and perhaps the ultimate -- tactical selling point of the bill.

It is this: the more painful the automatic cuts, the less likely they are to be invoked. Indeed, if the bill works in the civic-text manner as proposed by sponsor Sen. Phil Gramm (R-Tex.), the automatic cuts never would be invoked; the mere threat of them would be sufficient to force Congress and the president to do their job and reduce the deficit in a more rational way.

There are members of both parties who are going along with the bill because they see it as a form of legislation-by-indirection, a Trojan horse for bringing about two steps so sensitive no one dares legislate them directly: tax increases and Social Security cuts.

"This sets the stage for the grand compromise that almost everybody knows has to happen, sooner or later," said Rep. David R. Obey (D-Wis.). "We Democrats take entitlement cuts, they Republicans take defense cuts and we all go along with some new revenues . . . . There is already a broad consensus in the private sector for that tripartite attack, but it hasn't been ratified by the public sector because Ronald Reagan keeps telling the country that we don't have to take any bad medicine."

But Obey's tidy assessment of where it all might lead is not universally shared. "I have never seen us make a successful run at Social Security -- and I'm not sure we ever will," Rep. Bill Frenzel (R-Minn.) said.

Meantime, most members now believe that if the legislation is enacted, the automatic cuts would have to be invoked at least once, so everyone could feel the club.

That worries supporters. "We're going to create chaos in the short term," Panetta said. "We're playing with dynamite. When you stimulate crisis, you could lose control of it and you could damage the economy for years to come."

Whatever the motivations of supporters, and whatever the fate of the legislation, there is also a growing sense on Capitol Hill that members are trapping themselves, by their own rhetoric, into eventually doing something about the deficit.

"The whole agenda, the whole dynamic of this place has been fundamentally changed," Rep. Dick Cheney (R-Wyo.) said. "When you have virtually every House Democrat voting for some automatic sequestering of funds, it's a signal that a significant political shift has occurred."

Panetta agreed. "We all have to be deficit-cutters now. We have no place to hide anymore."