The head of the Strategic Air Command, seeking to overwhelm critics of the controversial MX missile, yesterday told Congress that the question of the new weapon's vulnerability was no longer an issue in part because "we have discovered that existing silos are harder than originally thought."

The SAC commander, Gen. Bennie L. Davis, also told the Senate subcommittee on strategic and theater nuclear forces that the "window of vulnerability" had nothing to do with the question of whether U.S. intercontinental ballistic missile silos could survive a Soviet nuclear attack.

"The whole question of a 'window of vulnerability' that was raised some years ago did not relate specificially to the vulnerability of missile silos," he said. "Instead, it had to do with the Soviet strategic nuclear forces overtaking and surpassing U.S. capability."

Questioned about another controversial decision facing the Reagan administration, Davis said he was "today" in favor of the United States continuing to abide by the provisions of the unratified strategic arms limitation talks (SALT) II treaty that expires at the end of this year. But he also indicated that President Reagan should keep his options open on extending its limits, which the administration plans to discuss with the Soviets at the Geneva arms control talks that begin next week.

For decades, U.S. strategists have argued that the vulnerability of "soft" U.S. missile silos to a growing Soviet first strike threat created an instability in forces between the United States and the Soviet Union.

Sen. Gary Hart (D-Colo.) took sharp issue with Davis' statement yesterday, saying it was the first time in three or four administrations that he had heard anyone argue that the "window of vulnerability" had nothing to do with the state of U.S. missile silos. "That was precisely what they were talking about," Hart said.

Arguing against ratification of the SALT II treaty in 1979, Paul H. Nitze, the administration's chief coordinator of the U.S. delegation to the forthcoming Geneva arms talks, made a major issue of the window of vulnerability, a phrase Reagan used frequently during the 1980 presidential campaign.

Five years ago, Nitze argued that by the early 1980s the Soviets would be able to knock out 90 percent of U.S. ICBMs with only one-fifth to one-third of their long-range missiles.

Davis deplored the fact that the focus of the debate over the 10-warhead MX had remained stuck on the question of its survival under a Soviet first-strike attack. The situation had changed, he said, because the survival of individual missiles "is better than we projected and can be further enhanced."

He ascribed this partly to improvements in the U.S. command, control and communications network and early warning system and partly to the "discovery" that existing silos were more resistant to attack than previously thought.

"Furthermore, research over the last year provides highly encouraging indications that we can build silos with hardness levels much greater than current designs," he added.

Echoing the president's MX missile report issued Monday, Davis also sought to convince Senate subcommittee members that the survivability of the U.S. nuclear force rested on all three legs of the "Triad" -- bombers, submarine- and land-based missiles -- and not on silo-based missiles alone.

He said that a successful simultaneous attack against all three was impossible, thus reducing the importance of silo vulnerability.