The Reagan administration is caught in a tightening political squeeze on arms control. Conservatives are pressing the President to publicize more fully new intelligence data indicating that the Soviet Union may be violating existing arms treaties more than is generally known.
The Pentagon recently warned that the Soviet nuclear-warhead arsenal now outnumbers that of the United States. And a senior defense official this week said that US satellites designed to check on Soviet missile testing are being jammed by the USSR.
From the opposite quarter, arms-control advocates are urging the President to stick to the tenets of the antiballistic missile (ABM) and unratified SALT II (strategic arms limitation) treaties. And they are pressuring him to revive dormant efforts to limit nuclear-weapons testing.
Much of this is coming from the Republican-controlled Senate - where a handful of arms-control resolutions were attached to the defense budget bill - and from arms-control experts who served under former Republican presidents.
Meanwhile, Washington's chief strategic-arms negotiator Thursday indicated that - for all the talk about superpower summitry - the administration has not softened its line on arms control. Edward Rowny said US strategic nuclear weapons ''will remain NATO's ultimate deterrent.'' And he rejected the argument that the US should be more forthcoming at Geneva, as administration critics have suggested.
Moscow has refused to set a resumption date for the talks on strategic (intercontinental) nuclear weapons, and Soviet negotiators walked out of the parallel effort at Geneva to reduce medium-range nuclear weapons. This walkout occurred late last year when NATO began deploying new Pershing II and cruise missiles in response to the Soviet buildup in SS-20 nuclear missiles targeted on Western Europe.
''One-sided cuts in our defense programs or failure to uphold alliance commitments would only reward the Soviets for their intransigence and make a return to the negotiating table less likely,'' Ambassador Rowny, a retired Army general, declared in a speech to the Royal United Services Institute in London. The Soviets, he added, ''are clearly testing western resolve.''
In Moscow Thursday, the official attitude also seemed to be tilting away from the possibility of a US-Soviet breakthrough. Kremlin spokesman Leonid Zamyatin said there had been ''no change in the position of the US to make a summit a real and feasible possibility.''
In his London speech, Rowny said that one of the ''most important shortcomings'' of the SALT II treaty was that it allowed the Soviet Union to deploy ''massive numbers of large, highly accurate nuclear warheads.''
Other experts, however, suspect that the Pentagon is raising fears about a ''warhead gap'' to fight arms control and promote new US nuclear weapons. In a report this week, the Arms Control Association (a private organization) surveyed recent Defense Department documents and assertions and concluded that questionable ''worst case'' assumptions were used to ''contrive'' a sharp inflation in Soviet warheads.
At the same time, however, authors William Arkin and Jeffrey Sands (who are working on a series of nuclear weapons databooks) admit that ''a substantial US lead in numbers of warheads can no longer be taken for granted.''
Such increased interest in superpower nuclear arsenals also is behind congressional efforts to limit weapons testing and not let the constraints of existing treaties be loosened. The House attached several significant amendments to defense and energy bills. Rep. Les Aspin (D) of Wisconsin termed this ''the greatest series of arms-control victories ever accomplished in the legislative arena.''
The Senate this week, by a 3-to-1 vote, urged the President to seek ratification of the Threshold Test Ban Treaty and the Peaceful Nuclear Explosions Treaty. Senators also asked Mr. Reagan to push for resumption of negotiations on a comprehensive nuclear test ban.
The Reagan administration wants to maintain the appearance of reasonableness and flexibility on arms control. But the clearest response from the administration to recent congressional pressure may have come from START negotiator Rowny, and it indicated little change.
''We must not appear to be overly eager for an arms-control agreement,'' he said. (The original text of his speech had said: ''We must be prepared to live without an arms-control agreement.'')
''If we want to be in a position to negotiate arms reductions with the Soviets, we first have to convince them that we have the will to match them in the absence of an agreement,'' Rowny declared.