President Carter's decision to build the MX block-buster missile came under heavy congressional attack yesterday, with Sen. Adlai E. Stevenson (D-ILL.) charging that the weapon "threatens the life of the SALT process itself."

Sen. Edward M. Kennedy (D.Mass.) warned that the MX, because it would be lethal enough to destroy Soviet missiles in underground silos, would give the Soviets more incentive to firee their weapons at the first sign of an American attack.

And Rep. Les Aspin (D-Wis.), a member of the House Armed Services Committee claimed Carter limited the nation's future strategic options by choosing the largest MX on the drawing board rather than a smaller version that could fit into Trident submarines as well as to deployed on land.

While politicians leveled these and other criticisms against the weapon, the Congressional Budget Office released a 135-page report saying it is not safe to assume that the Soviets would give up on building enough warheads to cover all the targets presented by a mobile MX system.

Stevenson and his anti-MX allies plan to meet with Carter to voice their concerns about the block-buster missile, which is designed to carry 10 war-heads of 335 kilotons each.

"The MX missile signifies that the United States is sacrificing the surest component of deterrence, the option to retaliate to a Soviet first strike by wasting Soviet cities and the Soviet economy," Stevenson said and pursuing instead a capability to knock out Soviet missiles before they can be fired.

"Being threatened by our MX," Stevenson reasoned, "the Soviets will very likely feel compelled to protect their own deterrent forces," partly by building more warheads than originally contemplated.

As a further hedge, Stevenson said in a speech on the Senate floor, the Soviets might decide to make their ocean-spanning missiles as the MX. "With a closed society and vast territoral expanses," he said, the Soviet Union would have an easier time hiding its missiles than would the United States.

"The future of arms control - with each side accepting land-moble systems, deliberate deception, first-strike accuracy, enormous payloads and pressures toward launch on warning - will be dim at best," he said.

"The president's decision on MX requires the supporters of the SALT process to reexamine" the strategic arms limitation treaty (SALT II), he said.

Kennedy warned that deployment of the MX would "lead the Soviets to rely on a launch-on-warning doctrine and even to consider moreseriously" launching preemptive strikes against the U.S. nuclear offense in a crisis.

Aspin, in a House floor speech said he could understand why Carter might have concluded that he had to build an MX to win to Senate approval of SALT II, but Aspin claimed the president chose the wrong version of the missile.

Rather than opt for the 92-inch MX blockbuster, Aspin argued, Carter should have ordered the 83-inch alternative missile, which could be based at sea inside submarines as well as on land in mobile launchers.

Aspin quoted testimony by William Perry, Pentagon research director, who stated the smaller MX would deliver all the punch sought by the Defense Department.

"There is noreason for going more than 83 inches in diameter," Aspin quoted Perry as telling the Senate Armed Services Committee March 7.

The White House confirmed last week that Carter had decided to build the largest MX%, 92 inches in diameter and weighing 190,000 pounds. The president did not elaborate on his reasoning, as was the case when he scrapped the B1 bomber. Also, the White House insisted that explanations Pentagon officials provided last week to reporters be given on a background basis, according to administration officials, ratherthan on-the-record.