Washington — At a critical moment in his foreign affairs program President Reagan will address a joint session of Congress next Wednesday to argue that the United States take stronger steps in El Salvador and Central America.
Some see the President's speech as a potentially decisive move in dealing with Congress on foreign issues.
Mr. Reagan has taken a stern line with the Soviet Union during his two years in office. Relations between the US and the USSR have deteriorated in a continuance of a trend that led away from the detente of 1972-75. Reagan campaigned and came to office saying that the Soviets had deceived other presidents, and his announced goal was ''to stand up to the Russians.''
Now a number of the President's initiatives are coming to the test at the same time in Congress. They include:
* El Salvador: Mr. Reagan wants the House to restore $50 million cut from his Central America that might extend even to Mexico. Congressional critics, on the other hand, argue that the US may become increasingly involved in El Salvador, as it did in Vietnam.
* Nuclear weapons freeze: Congress wants a role in proposed deployment of US intermediate-range nuclear missiles in Europe. That deployment is scheduled to begin in December if Washington and Moscow don't reach an agreement to limit such weapons. The House prepared an arms freeze resolution which the Senate almost certainly won't pass. Congress shows its uneasiness at the same time that freeze agitation is widespread in Europe.
* Arms violation: The administration comes closer to charges that the Soviets have violated the terms of the SALT II agreement. This treaty was never ratified by the Senate, but both sides informally agreed to adhere to its terms. Conservative Sens. Orrin G. Hatch (R) of Utah and Steven D. Symms (R) of Idaho demand an administration response to new charges of Soviet bad faith. The senators argue that Moscow understands only direct military pressure.
* MX missiles: Secretary of State George P. Shultz and Defense Secretary Caspar W. Weinberger lead the administration's drive to have Congress approve the new MX placement program. This would put 100 MX missiles in silos that now house Minuteman III missiles in Wyoming and Nebraska. On Thursday, members of the Joint Chiefs of Staff were scheduled to testify in favor of the new basing mode, which faces strong opposition in Congress.
* Libyan planes: Disclosure by Brazil that four Libyan transport planes landing for fuel in its territory carried arms and explosives, destined for Nicaragua and possibly for insurgents in El Salvador, intensifies debate. Brazil was told that the planes (three of them Soviet-made) carried medical supplies. It dramatizes and emotionalizes Reagan's charges that Caribbean trouble is Moscow-inspired. At the White House deputy press secretary Larry Speakes says the grounding of the Libyan cargo planes in Brazil was ''foolproof evidence'' of outside forces supplying rebels who are trying to destabilize the US-backed El Salvador government.
* Mexico: Secretary Shultz's recent visit to Mexico City left certain regional policy differences unresolved. Mexico's unemployment is high, its external debts are heavy, and migration of illegals into the US is increasing.
Some see a showdown over President Reagan's foreign policies in the accumulated problems now arising. His tough-talking approach has reversed a policy that has generally been followed since President Nixon. Writing on ''Reagan and Russia'' in the magazine Foreign Affairs (Winter 1982-83), Seweryn Bialer of Columbia University and Joan Afferica of Smith College argue that Reagan policies ''continue to display the characteristics of an ideological Crusade.'' At the same time, a nine-page analysis by Strobe Talbott in Time magazine (April 18), with a front cover question ''Arms Control: Making the Wrong Moves?,'' asks whether the US is following an appropriate policy. Citing what he calls a ''stunning repudiation'' by the Senate Budget Committee of a proposed Reagan defense budget increase, Mr. Talbott writes, ''The lawmakers seemed to be representing a growing discontent on the part of their constituents.''