Sen. Sam Nunn (D-Ga.) and his allies stood firm yesterday against White House pressure as the Senate neared a vote on their proposal to change the size and character of President Reagan's MX missile program.

Nunn, ranking Democrat on the Armed Services Committee, said last night that two days of negotiations with White House officials had produced "an impasse."

He said he would proceed with his amendment to cap MX deployment at 40 missiles, rather than the 100 sought by Reagan, and to continue producing additional ones for spares and flight testing.

As intense lobbying continued, the Senate resumed debate last night on Nunn's amendment to the Senate's fiscal 1986 defense authorization bill.

Majority Leader Robert J. Dole (R-Kan.), who said earlier yesterday that Nunn has enough votes to assure passage, insisted last night that the vote not be held until today.

"The president of the United States has an interest in this. He wants his point of view of expressed," Dole said as opponents of the amendment prepared to debate.

"The ball will be in the administration's court," Nunn said earlier in the day, adding that he might agree to deployment of more missiles if Reagan finds a less vulnerable way -- such as making them mobile -- to base them. The MX is now planned for deployment in Wyoming in silos once filled with smaller Minuteman missiles. Before resuming the MX debate, the Senate took a step toward breaking a 16-year moratorium on manufacturing nerve gas. With Vice President Bush presiding in case he was needed for a third time to break a tie on the issue, it voted, 50 to 46, against an amendment to eliminate $163 million requested for production.

Bush broke two Senate tie votes in 1983, only to see the House reject production of nerve gas and artillery shells and bombs to carry it. The House has done so for the last three years.

Sen. David Pryor (D-Ark.), the amendment's sponsor, argued that the United States has enough chemical munitions to kill everyone in the world 5,000 times. Breaking the production moratorium, he said, would intensify the arms race for no good reason.

Sen. John W. Warner (R-Va.) countered that the best way to deter the Soviet Union from using nerve gas is to confront Moscow with modern chemical munitions.

During the MX debate, Nunn likened Reagan to a man asked whether a bird in his hand is alive or dead. If the cap is imposed, Nunn said, the president can blame Congress for killing the MX, or he can portray the action as a prelude to finding a better basing mode or accelerating development of the small, mobile Midgetman missile.

"If the White House says, 'We've been robbed of a great bargaining chip, and the MX is dead,' " Nunn said, "it will become a self-fulfilling prophecy. It is in the White House's hands."

Nunn was interrupted at that point by a message requesting that he meet again with Reagan's national security affairs adviser, Robert C. McFarlane. "I'm being called off the floor by the fellow with the bird in his hand," Nunn quipped.

After their meeting, Nunn said McFarlane was speaking for Reagan in asking that the MX cap be raised to at least 50 missiles and that more be produced than the 12 that would be authorized under the controversial amendment.

"I am now at the end of the line on MX as long as it is deployed in vulnerable Minuteman silos," Nunn said.

For almost 10 years, the Air Force has said it must deploy 200 of the 10-warhead MXs to counter the Soviet Union's missile buildup.

President Jimmy Carter agreed to having 200, but Reagan, after several revisions, settled for 100 MXs in Minuteman silos in combination with the smaller Midgetman that would be moved around military bases to make it difficult for Soviet gunners to target.

Backers of the Nunn amendment contend that deploying 200 MXs would put a hair-trigger on nuclear war by confronting the Soviets with a weapon that could wipe out almost all Soviet land-based missiles unless those missiles were launched at the first sign of attack, perhaps even in response to a false alarm.

The proposed force of 40 MXs, according to Nunn and his allies, would not constitute a first-strike threat and would not destabilize the balance of terror.

Opponents of Nunn's amendment said they intend to argue that limiting deployment would undercut negotiators at arms-control talks in Geneva. Sen. Strom Thurmond (R-S.C.) said this is the wrong time to send the Soviets any message that might indicate weakening U.S. resolve.

The Nunn amendment is one of several controversial proposals that slowed Senate work on the record authorization bill. The pending measure would provide most of the $302 billion sought by the Defense Department for fiscal 1986, which begins Oct. 1.

The Senate decided to postpone until next month consideration of proposals to limit U.S. military activities in Central America and provide aid for rebels fighting the Nicaraguan government. Dole said the aid question will be debated in connection with the State Department's fiscal 1986 authorization.

The Senate voted, 68 to 30, against a proposal to delete $53.5 million authorized to bring the battleship USS Wisconsin out of mothballs. Congress has approved reactivation of three other World War II battlewagons.

Sen. John Glenn (D-Ohio) withdrew an amendment that would have required the Defense Department to return to the Treasury $4 billion it found as the Senate Armed Services Committee began markup of the authorization bill.

By voice vote, the Senate approved an amendment by William V. Roth Jr. (R-Del.) and Dennis DeConcini (D-Ariz.) to establish a 21-member bipartisan commission to recommend ways to improve Defense Department procurement. Commission members would be drawn from the Pentagon, the defense industry and Congress.