The Senate, breaking a four-month stalemate over arms control, moved toward a showdown vote today on a Democratic proposal to restrict testing and development of President Reagan's space-based, antimissile defense program.

The proposal, contained in the defense authorization bill for next year, would require congressional approval before Reagan could reinterpret the 1972 Antiballistic Missile (ABM) Treaty to allow expanded tests of the antimissile Strategic Defense Initiative (SDI) that has been dubbed "Star Wars" by critics.

The House has approved a similar provision in its fiscal 1988 defense bill, and Reagan has threatened to veto the defense measure if it contains SDI constraints.

While Republicans anticipated Senate passage of the restrictions, they argued they would win in the end because of the difficulties Democrats face in trying to muster the two-thirds vote in both houses needed to override a veto.

But this would force the president to veto a defense bill for the first time, a step Republicans were anxious to avoid. It would also present Reagan with what could be his most serious challenge so far from Congress on arms control, just at the moment that he hopes to seize the initiative on the issue with a U.S.-Soviet treaty to eliminate intermediate-range nuclear forces (INF). While the House has often defied him on arms issues, the more conservative Senate, in Republican hands until this year, has been reluctant to do so.

The Senate's moves yesterday toward a showdown came in the midst of a three-day visit by Soviet Foreign Minister Eduard Shevardnadze to discuss the INF agreement and other arms issues, prompting charges by Republicans that Congress was making concessions to the Soviets that they have been unable to win at the bargaining table.

The proposed SDI curbs would place "unilateral constraints on the United States and give a substantial concession to the Soviet Union at a critical juncture in the negotiations," said Sen. John W. Warner (R-Va.), ranking Republican on the Senate Armed Services Committee.

Democrats rejected the charge and retorted that Republicans were seeking a "blank check" for SDI in disregard of longstanding treaty interpretations and the Senate's shared responsibilities with the White House in the making of treaties and foreign policy.

Facing a vote to break their four-month filibuster against consideration of the measure, Republicans last week switched tactics, abandoning the filibuster but throwing another obstacle in the Democrats' path. They proposed a resolution opposing "unilateral concessions" and implying that any congressional action on the ABM-SDI issue would fall in that category.

Democrats counterattacked Tuesday with a resolution of their own that straddled the political traps laid by the Republicans and prompted another GOP filibuster and a stormy session that lasted late into the night. That impasse was broken only after Democrats angrily threatened to hold the Senate in round-the-clock sessions, suspending all other work, including confirmation hearings for Supreme Court nominee Robert H. Bork.

The Democratic resolution, which supported Reagan's negotiating efforts and opposed "unilateral concessions" by either the White House or Congress, was approved 92 to 1. The Republican proposal was then rejected, 59 to 35, with seven GOP senators opposing it.

In a preliminary vote last night, the Senate joined the House in asserting that a Soviet radar installation being built at Krasnoyarsk east of Moscow is a violation of the ABM treaty.

Democrats went along with the language, proposed by Sen. Dan Quayle (R-Ind.), who said it pointed up "a double standard" under which the United States restricts itself while the Soviets violate the treaty. But the Democrats contended it was a separate issue and has no relevance to today's ABM vote.

Action on the ABM-SDI issue could pave the way for passage of the $303 billion defense authorization bill for fiscal 1988, which includes $4.5 billion for SDI, an increase of nearly 25 percent over current spending. Some senators, including Nunn, have indicated they would favor cutbacks in SDI funding if the ABM provision is knocked out.

But Republicans are reserving the option to resume their filibuster, which could complicate final passage. Moreover, further arms control initiatives are possible, including a push for a nuclear test ban and for weapons limits to force resumption of compliance with the unratified SALT II treaty with the Soviets. Both proposals were approved by the House over opposition from the White House, and the SALT II proposal is considered to have a reasonably good chance of passage by the Senate.