For a while, it looked good for Rep. Joseph P. Addabbo (D-N.Y.) and those others in Congress who wanted to slash the Pentagon budget.

Then suddenly the same President Reagan who had been rattling sabers and promising to rearm America picked last Wednesday to make his big disarmament speech.

"Cost me 80 votes," Addabbo said of the speech, trying to explain why his amendments to kill the B1 bomber and MX land-based missile blew up on the pad Wednesday night, only hours after Reagan had offered to forgo deployment of medium-range missiles in Europe if the Soviets would do likewise.

"Maybe 50 votes," said Rep. Jack Kemp (R-N.Y.), who fought the cuts partly on the ground the president needed the B1 and MX to show the Soviets he was really serious about rearming if they did not agree to his proposals for disarming.

Whatever the reasons, the same House that had been warning the time had come to reduce spending for guns as well as butter flinched when it actually came to denying the president those highly visible superweapons. It voted down Addabbo's B1 and MX amendments 263 to 142 and 264 to 139 respectively.

Yet other evidence suggests the B1 and MX votes were not a gauge of members' true feelings about how much is enough for defense. The House Appropriations Committee a few days before the floor debate approved the MX by only two votes. And, right after the B1 and MX votes had been tallied Wednesday night, the same House almost smacked the Pentagon with a 2 percent cut in its research and procurement accounts.

"With a little more work," said Rep. Les Aspin (D-Wis.), "that amendment would have passed." It was voted down 202 to 197.

"They felt cross-pressured," said Aspin of House members in trying to explain the apparent contradictions in the voting. "They didn't want to deny the president something as visible as the B1 and MX, and risk looking soft on defense, but they also wanted to make economies.

"When you look at the record, the House usually goes along with the president on the big weapons. When the last president was against the B1, they were against the B1. When this president is for the B1, they are for the B1."

What does this say about Congress' approach to the key questions of national defense?

"Congress doesn't know what to do about national defense," Aspin said.

House Majority Leader James C. Wright Jr. (D-Tex.) agreed in another interview that the House would much rather reduce spending through general percentage reductions in a big, faceless account than zero in on highly visible individual programs with vocal constituencies. This is true of both civilian and military budgets, he said.

The $38.2 billion in domestic spending cuts that the House passed in June in the omnibus reconciliation bill would never have made it through if it had been examined on the floor program by program, said Wright. At the time, Rep. Leon E. Panetta (D-Calif.) complained about having to vote up or down on the lump of cuts called Gramm-Latta: "We are dealing with over 250 programs with no committee consideration, no hearings, no debate and no opportunity to offer amendments."

Another reason across-the-board cuts are easier to achieve in the Pentagon budget than specific ones is the big guns the Pentagon can roll up when its weapons come under attack. The arguments over Reagan's proposals for redressing the strategic balance showed that over the last several weeks.

No sooner had Reagan made his proposals Oct. 2 than Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee, normally a symbol of Pentagon strength on the Hill, attacked them. He said the president's recommendation to stuff the new MX into existing missile silos would not provide the reduced vulnerability everybody agreed was needed.

His House counterpart, Rep. Melvin R. Price (D-Ill.), warned that the president's recommendation to build 100 B1 bombers would run into heavy flak in Congress. Sen. Ernest F. Hollings (D-S.C.), who also usually votes with the Pentagon, said the nation could not afford to buy the B1 as well as the Stealth bomber and should scrap the B1 proposal.

As the debate heated up, the Air Force came to regard Defense Secretary Caspar W. Weinberger as more drag than lift for its causes, particularly the B1 bomber. Some generals nicknamed him "Mr. Ready-Fire-Aim" after he told the Senate flat out that the B1 bomber would not be able penetrate Soviet defenses after 1990 unless somebody wanted to order suicide missions. This would mean that the $300 million bomber, scheduled to start going into service in 1986, would be good as a manned penetrator for only four years.

As Hollings and other B1 opponents seized upon that testimony to try to delete about $1.8 billion in the Pentagon budget, the Pentagon brought in the heavy artillery.

Weinberger and Central Intelligence Agency Director William J. Casey wrote Congress a letter saying the B1 would be able to penetrate the Soviet Union into the 1990s.

Richard D. DeLauer, Pentagon research director, lifted the secrecy veil to tell the lawmakers in one secret session after another about the wonders of the B1 and the promise of the Stealth, exhorting them to support both. Weinberger sent Tower a letter promising to reexamine the idea of making the MX mobile to make it hard to hit.

Reagan called key lawmakers, telling them he needed the new weaponry so he could negotiate arms reductions with the Soviets from a position of strength.

Votes for making big cuts in the Pentagon budget and killing the B1, MX and other weapons melted under all this heat. The doubting lawmakers evidently felt outgunned and underadvised, and did not want to look weak on defense at a time the polls indicated the American people wanted to spend whatever was necessary to keep up with the Russians.

How long this mood will last is anybody's guess. Addabbo predicted it will be shortlived; that a recession will embolden the lawmakers to resume next year the cutting he started within his defense subcommittee this year. He lost the battles of the B1 and MX on the House floor, but his subcommittee's bill did end up being $3.4 billion less than Reagan had requested for the Pentagon for fiscal 1982.

And as he observed when asked what will happen next: "The battle continues."