Sen. Daniel Patrick Moynihan (D-N.Y.) may have put his finger yesterday on the reason President Reagan is so resistant to any further cuts in the defense budget.

"When it is cut further," Moynihan said, "it will be the Carter program."

Moynihan quipped that would amount to "continuity of government, and that's not altogether bad." But it would be an embarrassment to Reagan, who denounced the Carter military budget as too little during the campaign.

By Moynihan's arithmetic, Reagan's proposal to cut the Pentagon budget by $13 billion in spending and $27 billion in budget authority over the next three years will leave the Reagan proposals only 2.6 percent higher than Carter's last five-year projection. Additional cuts by Congress would quickly shave that.

Chairman Pete V. Domenici (R-N.M.) of the Senate Budget Committee, trying to stave off more cuts in the Reagan defense budget, said by keeping it intact there would be $85 billion more available than Carter had proposed for the next five years.

"Only in Washington," said Defense Secretary Caspar W. Weinberger as he stood beside Domenici outside the hearing room, "could anyone say that was insignificant."

Weinberger sought to put his budget off-limits to further cutting by Congress, declaring the administration has done "all that can be done." He said administration officials had reduced a number of programs that are "slightly less necessary than some others."

In the closed session of the Senate Budget Committee, he detailed only cuts proposed for the fiscal 1982 defense program, talking in general terms about plans for economies in fiscal 1983 and 1984. A number of lawmakers are expressing dismay about the lack of specifics beyond fiscal 1982.

Pentagon officials said the arguments over what to cut in fiscal 1983 and 1984 are still raging inside the administration, with the five-year blueprint subject to radical change by the decisions Reagan makes on a new bomber and deploying the MX land missile.

Even for 1982 the administration's proposed cuts are constantly being revised. But here, according to sources, are the major ones planned for fiscal 1982 in budget authority:

Fairchild A10 antitank plane. The administration plans to reduce the program from 60 to 20 aircraft to save about $350 million. This could cost jobs in Fairchild's plants at Hagerstown, Md., and on Long Island.

McDonnell-Douglas KC10 cargo plane. Virtually killed through a cut of almost $500 million. The Carter administration had portrayed this plane as vital to helping the Air Force respond to crises in the Persian Gulf.

French-German Roland antiaircraft missile for the Army. Canceled to save $477 million. This is almost certain to provoke protests from France and Germany that the United States is failing to live up to its pledge to make weapons buying a two-way street.

British JP233 munitions for attacking airfields. Deleted from research budget to save $60 million, an action likely to draw stiff objections from London.

Army M1 main battle tank. Production stretched out to save $275 million.

Ammunition. Planned purchases reduced by $160 million, delaying the day when a 60-day supply will be stockpiled in Europe and the Pacific theater.

Navy ships. One FFG7 frigate and three fast cargo ships, one new one and two conversions, deleted from the shipbuilding account.

Air Force B52D. Some of these old bombers will be retired earlier than planned to save $62 million.

Military constuction. Army cut by $169 million, costing it a new hospital in Hawaii; Navy by $94 million, encompassing a delay in bringing the Trident submarine base at Kings Bay, Ga., into service; Air Force by $24 million.