The foreign policy establishment so scorned by President Reagan on coming to office has now been enlisted by the White House in a last-ditch effort to save the administration's strategic nuclear weapons program.

Members of the bipartisan, mostly eastern and unofficial network have been brought into the work of the president's Commission on Strategic Forces, which was created to find a politically acceptable deployment plan for the MX after Congress, in a rare rebuff of a sitting president on a major weapons question, voted to shelve the missile last year.

Those involved include such familiar figures as Lloyd M. Cutler, White House counsel in the Carter administration; Harold Brown, President Carter's defense secretary; Donald Rumsfeld, defense secretary in the Ford administration, and Henry A. Kissinger, Nixon-Ford administration secretary of state.

Cutler, Brown, Rumsfeld and Kissinger are now consultants to the commission, whose chairman is Brent Scowcroft, former Kissinger aide in government and now a Kissinger business associate.

Their fear, some of these participants say, is that a longtime national consensus on both halves of the nuclear equation--arms control as well as weapons procurement--has been shattered by the mainly Republican resistance to the Carter strategic arms limitation treaty in 1980 (SALT II) and the mainly Democratic vote in the House against production funds for the Reagan MX last year.

Another defeat for the president on the MX this year would "send a terrible signal to the rest of the world," according to Cutler, lawyer and longtime Washington insider who was a leading proponent of SALT II.

"The continuity of 10 years or more on weapons and arms control programs" will be endangered if Congress votes down MX missiles without offering "a policy to replace them," he said in explaining his participation.

The leading administration official on the commission, senior National Security Council staff member Thomas C. Reed, uses almost the same language in describing the commission's purpose. There is more than just the MX on the table, he says. "It is terribly important to rebuild a bipartisan consensus on defense and national security that will stick from administration to administration."

This is unaccustomed talk from an administration that came to office bristling on defense questions and contemptuous not just of the national security policies of the Carter administration but the detente of the Nixon-Ford years as well.

The Scowcroft commission, including vice chairman Reed, has also made what one participant describes as "impressive and sincere" overtures to congressional Democrats, including some liberals.

In a further sign of a newly accommodative spirit, Defense Secretary Caspar W. Weinberger, symbol of the administration's continuing hard line on defense, has not been part of the commission's deliberations. One administration official said that Weinberger has assented to this because he recognizes that he has become a source of controversy on Capitol Hill.

"We can't let it be a vote of confidence on the defense budget or a vote of confidence on Cap Weinberger ," this official said. "It has to be a vote on the strategic modernization forces."

When the commission first began work, it was no secret that Reed and others expected quick endorsement of a so-called deceptive MX deployment plan in which 100 missiles would be hidden among 300 super-hardened silos.

But that variation on the administration's defeated "Dense Pack" basing proposal--bunching the MXs so that incoming enemy missiles would blow each other up in a process called "fratricide"--met with a chilly reception in first soundings on Capitol Hill.

"Both proponents and opponents have come to realize," said one House Democrat, "that any variation on that [Dense Pack] theme is doomed to defeat," and that a second defeat on the MX could spill over into other areas, threatening both the buildup and control sides of administration policy.

"The spirit has changed" at the White House, was the way one arms control expert put it. "They are more open-minded."

Reed says that the commission will take as long as necessary to do its work and that the search for consensus means its recommendations will go to the Hill only when the leaders there agree they should be taken up.

The recommendations will also apparently have an arms control component.

The commission's role has grown from simply solving the problem of how to base the MX to "rebuilding a bipartisan consensus for a sensible strategic policy which both Congress and the president can support," according to one participant in the commission's new deliberations.

"We are trying to develop an integrated arms control and weapons procurement program to reach that goal," he said.

That program has the following tentative components, according to interviews with commission members, consultants to the commission, members of Congress and administration officials:

Prompt approval of production funds for the MX and deployment beginning in 1986 in existing Minuteman III silos in Wyoming. This is considered a must if Reagan and defense conservatives in Congress are to support the plan. As one administration official said, prompt deployment of the MX is the only "non-negotiable" element of the White House position.

Many middle-road Democrats oppose the MX, but the argument will be made to them that MX deployments will be held below a number that would threaten the Soviets with a first strike, either by keeping the missiles below 100 or reducing the number of warheads on each missile to six instead of the currently planned 10.

Long-term research on a small, single-warhead ICBM that would be deployed in a mobile mode to help its chances of surviving attack. There is strong support on Capitol Hill for the small missile, and such a missile figures especially in an arms reduction plan that has been worked up by Rep. Albert Gore Jr. (D-Tenn.).

Gore has circulated his proposals not just among his Hill colleagues but also within the administration and even to Soviet officials. His ideas are being considered by commission members.

At the same time, several conservative strategists, such as arms negotiator Paul Nitze, also have proposed turning away in the 1990s from big missiles toward smaller, more mobile ICBMs.

Revising the Reagan arms control reduction proposal, called START, to take into consideration the smaller missile. How many such missiles is the question.

The current Reagan proposal calls for reduction in the number of Soviet and U.S. missile launchers to 850 each, and warheads to 2,500. To accommodate the small ICBM idea, the launcher limit would have to be removed and replaced by limits on the total nuclear payload each side could have on its missiles. Thus, one country could have many more missiles than the other but both would have the same nuclear destructive power.

Assuming that the administration and the congressional leaders are able to work out a technically acceptable package, Reed said it would go to the Hill as a bipartisan consensus on strategic modernization supported by the president.

It would, in effect, stand outside the defense budget even though it is part of it, much as the Social Security compromise stood on its own feet independent of the budget.

The commission has asked the Air Force to do a series of studies relating to the small missile. In addition, proposals for quick development of such a missile, such as defense contractor Martin Marietta's plan to expand the range of the Pershing II from 1,000 miles to 8,000 miles by adding a third stage, will be considered.