President Reagan has said in a letter that if Congress does not approve his MX missile there will have to be "a reassessment" of his strategic arms reduction proposals made to the Soviet Union in Geneva.

The president laid out his views in a letter this week to Rep. Jack Kemp (R-N.Y.). Kemp had earlier urged Reagan to consider suspending U.S. participation in the strategic arms reduction talks (START) and related negotiations over European-based missiles if Congress did not support both the long-range MX missile and the shorter-range Pershing II missile meant for deployment in Europe.

Funds for both were curtailed in last year's lame-duck session.

Reagan in his response to Kemp emphasized that his START proposals for deep reductions in the long-range missile arsenals of both sides were based on the "assumption" that the new MX missile would actually be deployed.

Administration officials indicated yesterday that this was meant as a warning to MX critics that if they kill the missile they could also cause a "very lengthy" reassessment of the U.S. position at the Geneva talks and thereby delay the arms control process, which many MX opponents support.

Kemp wrote the president as Congress last month was rejecting administration requests for $988 million to begin production of the MX.

That negative vote came largely because the lawmakers lacked confidence in the administration's proposed "Dense Pack" basing plan for the missile.

The administration is now restudying the basing question.

The president's letter also contains some of the clearest language yet used by the White House to make the point that the MX missile is not a so-called bargaining chip in the sense that it is not meant to be given away in negotiations with the Soviet Union.

The START negotiations deal with intercontinental missiles and bombers, while the Intermediate-range Nuclear Force or INF talks attempt to limit shorter-range weapons based in Europe.

In his Jan. 4 letter, which administration officials made available, Reagan gave no indication that he would break off the talks, and aides said that is unlikely.

The president wrote that he is "committed to seeking deep reductions to equal and verifiable levels of nuclear arms" by the two superpowers.

But he added that the U.S. proposal at START for sizable reductions in missiles and warheads on both sides "is based on the assumption that our force structure will include MX 'Peacekeeper' ICBMs in a survivable basing mode. We certainly could not accept a situation wherein all of the Soviet missiles permitted were recently deployed modern systems, while ours were all far older."

Reagan then invoked this assessment by chief U.S. START negotiator Edward L. Rowny:

"There is no question that a failure to fund and deploy the MX would have a serious impact on our future force structure and would handcuff our negotiators and require a reassessment of our START proposals."

Administration officials have claimed that MX is important for bargaining purposes because if MX is killed the Soviets have no reason to reduce their missiles.

On the other hand, if the Soviets will reduce their missiles the United States could agree to limit the number of MXs eventually fielded.

Many lawmakers who voted against the MX are not necessarily opponents of the missile itself but are frustrated with administration failure to come up with a plan for deploying it in a way that will keep it safe from attack, as the administration promised it would do in the campaign.

The White House, however, appears determined to press for at least production of the missile, no matter where it is ultimately based, believing that it is essential for defense and arms control plans.

Today, a new 11-member, bipartisan commission established by Reagan on Monday to take another look at how to base the MX will meet for the first time at the Pentagon.

The advisory commission was established after Congress, in rejecting the MX funding request, directed the White House to review all alternatives and come back with another recommendation after March 1.

In his letter to Kemp, Reagan took note of this requirement but said "we are confident that when the Congress has further reviewed these programs and their vital importance to our goals of effective deterrence and meaningful arms control, they will approve the funding and deployment schedules required."