One of the few people in Washington who professes to know exactly what the Gramm-Rudman deficit-reduction amendment would do to federal spending is Defense Secretary Caspar W. Weinberger.

Weinberger agitatedly told President Reagan and the Senate Republican leaders that Gramm-Rudman could cut the Pentagon budget drastically and "destroy" the administration's arms buildup.

That is exactly the reason, of course, why so many liberal Democratic senators voted for the automatic budget-cutting bill -- although they could not be dead certain that the rain would fall alike on the Pentagon and the poor.

Weinberger, who is extremely protective of his swollen purse, apparently takes no comfort from the general belief that Pentagon contracts already signed will be exempt from the guillotine built into the bill. If the stated goal of a $36 billion annual deficit reduction is not reached, the president will be required to cut across the board. But in recommending reductions, the sponsors explain, the president would have to take into account whether the cost of penalties and closing obligations would exceed the savings from canceling a contract.

The law is as ambiguous about defense spending as it is about domestic programs. A House-Senate conference committee, one of the largest ever assembled on Capitol Hill, is trying to define which projects are "sequestered" against cuts and which are "controllable." At a recent session, for instance, members were expressing uncertainty over the status of Medicare.

The most conspicuous "uncontrollable" program is Social Security. Rudman and coauthor Sen. Phil Gramm (R-Tex.) thought it should be put on the table with everything else, but they were talked out of this conspicuously dangerous exercise by the Senate Republican leadership. All political benefits of the budget cuts would be lost, they were warned, in the howl that would come from House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) that Reagan was once again beating on the elderly.

Even so, Rep. Anthony C. Beilenson (D-Calif.), noting that the dragon of the senior citizens, Rep. Claude Pepper (D-Fla.), was out of the room, suggested that the enterprise would have much more stature if Social Security recipients shared in the general sacrifice.

Sen. Lloyd Bentsen (D-Tex.) said he was "intrigued" by the suggestion that Social Security be removed from the "untouchable" category. He said he found it difficult to "see any equity and fairness" in cutting programs for poor mothers and babies while giving full measure to Social Security, which in some cases goes to the fairly well-off.

But Rep. Henry M. Waxman (D-Calif.) predicted darkly that the "president will decide not to cut defense and keep the sword of Damocles hanging over poor people."

Gramm-Rudman is the new Trojan horse inside the gates of Washington's consciousness. No one can define the beast. Some regard it as a villainous contraption designed to divert attention from the fact that the bill to which it is an amendment has raised the debt ceiling to a staggering level above $2 trillion. Others see it as unconstitutional since it cedes congressional powers to the presidency. To others, it's a message scrawled in lipstick on a mirror: "Stop me before I kill again."

It is, indisputably, an abject and humiliating confession by Congress that it simply cannot cut the deficit. After all, as Sen. Pete V. Domenici (R-N.M.) pointed out to the conferees, "It wasn't somebody out there in heaven who voted the defense budget. We voted for it. It wasn't forced on us."

Sen. John H. Chafee (R-R.I.) calls Gramm-Rudman "the worst thing, except for anything else." Sen. Daniel Patrick Moynihan (D-N.Y.) offered this elegant summation: "A majority of Democrats agreed to dismantle the domestic policies of Franklin D. Roosevelt, in return for a majority of Republicans agreeing to dismantle the defense policies of Ronald Reagan."

But Rep. Tony Coelho (D-Calif.), chairman of the House Democratic Campaign Committee, thinks that Democrats can support an amended Gramm-Rudman, however heavy their hearts that Republicans will get credit for at least addressing the deficit monster.

"Our domestic programs are losing anyway, and defense never loses," Coelho said. "Gramm-Rudman means the argument will be framed differently. Now it won't be funding a new high school against raising the deficit. It will be the new high school versus another MX. When it's put to them that way, the country will go for the school."