President Reagan's arms advisers now are saying there is no practical way to protect the MX missile to the point of closing the "window of vulnerability" Reagan campaigned against so vigorously in 1980, it was learned yesterday.

Senators said this shift in position came through loud and clear as they were briefed on the recommendations to be made Monday by Reagan's special advisory commission on the MX.

It will urge the deployment of 100 MX missiles in existing Minuteman silos in Wyoming, starting in 1987, plus a crash program to build a new smaller, mobile and less vulnerable missile by 1991.

Retired Gen. Brent Scowcroft, chairman, and other members of the panel briefing the senators said they had concluded that there is no practical way to harden existing Minuteman silos so they would make the MX invulnerable, that the MX will be too big and heavy to be mobile and that the Minuteman silos are spaced too far apart to make an antiballistic missile defense feasible.

In short, after more than 30 studies of every conceivable kind of basing scheme, this newest presidential advisory panel is saying that an invulnerable MX cannot be achieved.

This judgment flies in the face of the hot rhetoric Reagan used in 1980 against President Carter's so-called race track deployment plan for MX and the "window of vulnerability" that the Republicans said had opened up under the Democrats.

And if the MX will not be invulnerable it may be harder to sell to a skeptical Congress.

The vulnerability window, as the MX commission described the situation to about half the Senate yesterday behind closed doors, would remain open until the 1990s. Early in that decade the small missile, nicknamed Midgetman, would be deployed on Air Force bases and hauled along roads periodically to make it hard for the Soviets to target or hit.

The new MX basing plan, if Reagan embraces it, will be his fourth. The first three flopped, and last winter an impatient Congress voted to hold up production funds for the missile until Reagan could find it a home. It was that vote that led Reagan to turn to a commission.

In an effort to reassure senators, panel members at the briefing reportedly said there is little chance over the next several years that the Soviets could knock out both U.S. missiles and bombers in a surprise strike.

If the Soviets fired submarine missiles against the bombers first, the briefers said, the MX and Minuteman missiles could be launched before enemy warheads rained down on them. If the Soviets fired intercontinental ballistic missiles first, the briefers added, the United States would have enough warning to get bombers off the ground before they could be destroyed.

Defense Secretary Caspar W. Weinberger apparently anticipated a congressional backlash against the idea of placing MX missiles in Minuteman silos that have been declared vulnerable to Soviet attack. He sent letters a few days ago to members of Congress to move the focus away from an invulnerable land missile.

"The tendency in the past," Weinberger wrote in a letter obtained by The Washington Post, "both in the Pentagon and on the Hill, has been to over-focus on the basing issue and disregard the real question as to the need for modernizing the ICBM force. The ICBM vulnerability problem is only one of the myriad of strategic issues that requires serious examination by the Congress before casting a decisive vote this year on" future arms.

Weinberger urged the members of Congress to attend Air Force briefings to learn why a new land missile was vital for the national defense, invulnerable or not.

Sen. Mark Andrews (R-N.D.) said that he and Sen. Daniel Patrick Moynihan (D-N.Y.) warned members of the MX commission that their recommendation threatened to make it an "acceptable" strategy to fire U.S. missiles at the first warning of attack. "This presents the potential for a worrisome move toward the hair trigger," said Andrews in warning that missiles might be fired in response to false alarms. He added that a launch on warning policy was one "that we have wanted to stay away from."

Asked if he thought Congress would go along with putting MX missiles in Minuteman silos, Andrews replied, "They're making sunbeams from cucumbers."

He said that the commission is recommending deploying the MX in Minuteman silos only from 1987 or 1988 until 1991, when the Midgetman is scheduled to be deployed. "That's $20 billion for a three-year program," Andrews said.

"If these Minuteman are targetable by the Soviet Union, and if you have 300 warheads on 100 Minuteman missiles vulnerable now," he asked, "where you do gain by making a thousand warheads vulnerable" by putting 100 of the 10 warhead MX missiles in the same Minuteman holes? "That's Mickey Mouse.

"If we need to have a land-based missile option," Andrews continued, "then mobility is the key to survivability. Anything less than that is fooling ourselves, or worse, moving toward a first strike acceptability." The administration, Andrews said, "has got a lot of selling to do."

Andrews said that former secretary of state Alexander M. Haig Jr., a member of the MX commission, was among those who briefed the Senate yesterday. Haig argued that European allies would be disinclined to accept the new U.S. Pershing II and cruise missiles in their countries if the United States refused to deploy a new land missile on its territory.

Other briefers, who included James Woolsey, undersecretary of the Navy during the Carter administration, stressed that the MX was an essential bargaining chip in arms control negotiations with the Soviets.

"I think what we're embarking on," Andrews said, "is a long and, I hope, in-depth debate on just where we are going" in strategic weapons policies.