West European governments have welcomed President Reagan's new interim arms control proposal. It may take some of the heat off them. But the proposal contains only the skeleton of an idea. So many crucial details would have to be negotiated with the Soviets before that idea could take shape that an arms control agreement with Moscow may still be many, many months away.
To begin with, the President proposed no numbers of missiles for the two sides to station in Europe. To arrive at those numbers is likely to require hard bargaining, not only with Moscow but also within the administration itself. The State and Defense Departments still have a tendency to take a different approach to the Euromissile talks in Geneva.
But Assistant Secretary of Defense Richard N. Perle, who has played a key role in shaping the administration's arms control strategy, denied there were significant differences between the two departments regarding the President's new proposal. The State Department at first wanted more concreteness in the proposal - namely numbers. But Mr. Perle told the Monitor that by proposing reductions which did not yet include numbers of warheads, Reagan has ''put the burden on the Soviets and let the debate focus on our flexibility.''
In a televised statement made from the White House Wednesday, President Reagan unveiled his long-awaited proposal for an interim agreement with the Soviet Union. Under the President's proposal, the United States would substantially reduce its planned deployment in Europe of Pershing II and ground-launched cruise missiles, provided the Soviets reduced their intermediate-range missiles already based in Europe. The two sides would be required to come out with an equal number of warheads.
Administration officials believe that deployment of Pershing II missiles, which can reach the Western part of the Soviet Union in 12 minutes, will provide an incentive for the Soviets to negotiate.
In his statement, President Reagan declared that the Soviet Union had failed to offer any serious alternative to his so-called zero-zero option of November 1981, which called for the elimination of all land-based intermediate-range missiles in Europe.
The President said that Paul Nitze, the US ambassador at the Geneva negotiations with the Soviets, informed his Soviet counterpart of the new proposal. As a gesture designed to indicate its seriousness, the administration also suggested that negotiations resume, after a recess, several weeks earlier than originally planned. The Soviets agreed, and the talks are scheduled to reopen May 17.
But even if the Soviets accept the Reagan idea in principle, much negotiating would have to be done:
* The Reagan plan calls for effective verification of any agreement, always a difficult issue to negotiate with the Soviets.
* It insists that British and French intermediate-range missiles not be included in the talks. Administration officials argue that the British and French missiles are mostly submarine-based and are not designed to provide the ''nuclear umbrella'' which is needed to protect West Germany from any Soviet attack. The Soviets say these missiles must be taken into account.
* Reagan insists that all the Soviet SS-20s placed east of the Urals, mostly in Soviet Asia, be included in the negotiation. The Soviets have not agreed to this. Of some 350 SS-20 launchers, about 100 are east of the Urals, according to US sources. The SS-20s are mobile, and could easily be diverted from Asian to European targets.
* The Reagan proposal calls for no adverse impact on NATO's conventional defense capability. But according to administration officials, the latest Soviet proposal would nearly eliminate from the European continent US aircraft considered indispensable to NATO's conventional defenses. The Soviets argue that many of these are ''dual capable'' aircraft which can be used both for conventional defense and nuclear attacks on the Soviet Union.
Some critics see the administration's decision to exclude specific numbers of warheads from the new interim plan as a possible complication in the talks. But John Newhouse, a former arms control official in the Carter administration, argues that lack of specificity could be an advantage. As Mr. Newhouse puts it, a good negotiator ''usually doesn't make his best offer in the newspapers.''
Last year, Ambassador Nitze suggested to his Soviet counterpart, Yuli Kvitsinsky, a limit of 75 missile launchers for each side, with only cruise missiles being deployed on the American side. That would have solved the Pershing II problem for the Soviets. But Mr. Kvitsinsky's superiors were reported to have ultimately rejected the Nitze approach. Defense Department officials and others in Washington raised objections as well.