Defense Secretary Caspar W. Weinberger has sent a letter to Congress underscoring the Reagan administration's interest in determining if anti-ballistic missiles are the long-sought answer for protecting the MX land missile.

Pentagon officials have conceded that ringing the MX with ABM batteries would require breaking or amending an existing treaty with the Soviets which limits the deployment of protective missiles.

But, in identical letters to the House and Senate Armed Services committees this week, Weinberger protested a $468 million House cut in ballistic missile research, declaring it would hamper the Pentagon's exploration of that option for protecting the MX. Weinberger wrote:

"The $468 million reduction by the House will seriously impact the active defense option for survivable, longterm MX basing . . . . The drastic cutbacks in our BMD Ballistic Missile Defense program that followed signing of the ABM treaty have already resulted in BMD development lagging behind MX development."

His letter makes clear that the administration wants the MX and protecting missiles developed in tandem in case they end up being deployed together.

The House Armed Services Committee, though it has championed the MX, said it was imposing the $468 million cut in Weinberger's fiscal 1983 ABM request because it would not make sense to spend that much money before the administration decides how and where it wants to base the MX.

The committee also said that the right hand does not know what the left hand is doing in ABM research, declaring that "Army witnesses were not aware that $109 million had been included in the Air Force MX missile budget request to investigate a defended, deceptive basing mode for the MX missile."

The Senate committee, in approving the full $870.6 million requested, said the ABMs "could be indispensable to a survivable MX basing system."

The Carter administration recoiled from the idea of breaking the ABM treaty and recommended instead that the MX be protected by being kept on the move in western valleys. President Reagan scoffed at that idea and ordered the Pentagon to explore alternatives, none of which is without its problems.

One Pentagon argument is that breaking the ABM treaty would not only protect the MX, but also enable the United States to overcome the advantage provided by the Soviets' superior civil defense program. However, Reagan administration officials have not decided whether to extend, amend or scrap the ABM treaty, negotiated in 1972.

If Reagan should opt for ringing the MX with ABMs, the cost of the protective missiles would be "at least $10 billion and could be as much as $20 billion to $25 billion," according to the Pentagon.

Weinberger, in seeking restoration of the ABM money in his 14-page letter, offered no offsetting reductions. Instead, he asked the conference of House and Senate members now hammering out a compromise fiscal 1983 defense authorization bill to restore most of the big cuts imposed by either the House or the Senate.

His letter suggests that he does not feel obliged to make the Pentagon budget fit under the fiscal 1983 ceiling that Congress has already set for national defense. Reagan has said he does not feel bound by the defense ceilings in that resolution for fiscal 1984 and 1985.