The 98th Congress returns to work next week to deal with major foreign policy issues in an atmosphere of confrontation with the administration and confusion in its ranks.

So what's new?

If President Reagan was elected with a mandate to do what he wanted in the international arena, that message never reached Capitol Hill. He's meeting the same resistance on El Salvador and the nuclear freeze issue that President Carter got on Panama and SALT II.

In the Republican-controlled Senate the Foreign Relations Committee slashed in half his $60 million request for military aid to El Salvador. An Appropriations subcommittee gave him the full amount, but attached a list of uncomfortable conditions.

In the Democratic-controlled House the Foreign Affairs Committee approved a nuclear freeze resolution that calls on Reagan to change his strategic arms control proposals in midstream. It is likely to pass the full House, although a variety of conflicting amendments could be attached.

In the end, the administration may prevail on both issues, as it did in its hairbreadth 1981 victory on the sale of airborne warning and control system (AWACS) aircraft and equipment to Saudi Arabia, but not without entreaties, compromises, negotiations, indirect threats and a good deal of sweat. In his recent speeches attacking the nuclear freeze and defending his defense budget Reagan has shown himself to be up for the fight.

Tension between the president and Congress is built into the American system, but it is only in the last decade that Congress has asserted itself forcefully and continuously in the international arena. Reagan is but the latest presidential victim of this post-Vietnam era of congressional activism on foreign policy.

Gone are the days when Capitol Hill let the downtown crowd alone wear the striped pants, and, judging by the epidemic of second-guessing that has swept through the 98th Congress in its first three months, that era is unlikely to return soon.

"We have a role to play," said Sen. Charles H. Percy (R-Ill.), chairman of the Foreign Relations Committee. On El Salvador, he added, "I don't think the administration has any choice but to listen to us. They can't spend money we don't appropriate."

Reagan, however, has tried. More than 65 percent of American military aid to El Salvador--some $80 million since January, 1981--was drawn from emergency funds controlled by the president outside the direct authorization and appropriations process.

Now, to avoid floor fights, the administration wants to divert to El Salvador $60 million earmarked for Morocco in the fiscal 1983 budget, while acknowledging that it will later request new funds for Morocco.

Under a strict interpretation of the law, the administration must only "notify" congressional Appropriations subcommittees of such "reprogramming" requests. But the committees now claim the right to attach policy conditions to the request or even veto it.

The Senate Foreign Relations Committee also has asserted authority over reprogramming, based on a somewhat vague 1981 letter from Undersecretary of State James L. Buckley on another reprogramming request.

Last week 82 House members signed a curt letter from Rep. Michael D. Barnes (D-Md.), chairman of the Foreign Affairs subcommittee that oversees El Salvador, warning Reagan not to make "end-runs around Congress through misuse of discretionary authorities . . . . "

Reagan has charged that the nuclear freeze resolution would undermine arms negotiations with the Soviets. But administration allies warn that if the president doesn't consult more with Congress on arms control an eventual treaty might have difficulty winning Senate approval.

With consultation, early political problems can be defused, members of Congress say. Already the Senate Foreign Relations Committee has balked at Reagan's nominee for head of the Arms Control and Disarmament Agency, Kenneth L. Adelman.

Tensions between the executive and legislative branches have increased steadily since the Vietnam war, fundamentally altering the conduct of American foreign policy.

"The Vietnam war changed it," said Percy. "That burned Congress badly. That's why we passed the War Powers Act to place restraints on the president."

Under that 1973 resolution, the president must notify Congress if U.S. troops are sent into hostile situations abroad and must withdraw them within 60 days unless Congress authorizes them to remain.

Since then several laws have curtailed the president's freedom to conduct foreign policy, while expanding Congress' role: the 1974 Turkish arms embargo in response to the invasion of Cyprus; the 1974 Nelson-Bingham amendment allowing Congress to veto arms sales; the 1974 Hughes-Ryan amendment requiring congressional notification of covert operations by the CIA; the 1976 Clark amendment prohibiting U.S. involvement in military operations in Angola without Congress' permission, and the 1978 Nuclear Non-Proliferation Act forbidding the export of nuclear materials to nations without adequate safeguards. Foreign aid bills grew "barnacles," as the diplomats call them. Besides restrictions on aid to at least 30 countries, there were general bans against nations that expropriated U.S. property, harbored terrorists, violated human rights or failed to pay their U.N. dues.

With the war in El Salvador, Congress is becoming more involved in detail than ever. The president already has been required to certify progress in land reform and on specific human rights cases. And now, if the Senate Appropriations subcommittee conditions hold, he must promise not to send more than 55 military trainers and must make progress toward "unconditional dialogue" with the guerrillas.

Rep. Jim Leach (R-Iowa) said that, by focusing on human rights, congressional activism in foreign policy has "generated an image overseas of the American people as sensitive and responsive . . . . The multi-voiced concerns of Congress cushion the blows meted out by administrations motivated by short-term political advantage."

But Sen. John G. Tower (R-Tex.), chairman of the Armed Services Committee, argued in a magazine article last year that "The balance between Congress and the president has swung dangerously to the legislative side . . . . We may well be in a situation today which is analogous to that of the late 1930s, when America's inability to play a more active role in world affairs helped permit the Axis to realize its objectives without serious challenge."

Tower added that 535 "congressmen with different philosophies, regional interests and objectives in mind cannot forge a unified foreign policy that reflects the interests of the United States as a whole."

Or, as one high-level U.S. diplomat put it a few years back, "Involving Congress in foreign policy is like having 535 ants sitting on a log floating down a turbulent river--each one thinks he's steering."

Sen. Mark O. Hatfield (R-Ore.), a key figure in last week's fight over El Salvador, said, "Congress is in a very difficult role. We don't conduct foreign relations, but we influence it. We're mostly nudging. It's not as if we can come up with an alternative and replace existing policy with it."

Nonetheless, while the system is at times awkward, the executive branch has learned to use the congressional counterpoint to its own advantage.

The "much-maligned" legislative veto is rarely used, but it "serves as a club in the closet," according to the Carnegie Endowment's I.M. Destler in Foreign Policy magazine. "The threat forces prior consultation, information sharing and anticipatory adjustment by an administration before it has made a firm decision and staked its credibility on it."

"The executive branch has become very good at using Congress as the fall guy," said Frederick S. Tipson, chief counsel of the Foreign Relations Committee. "It tells foreign countries, 'If you don't come around, you'll have to deal with Congress.' "

Administration officials have acknowedged that the certification requirements it so strongly resisted for El Salvador aid have helped it push the Salvadorans toward changes.

As Secretary of State George P. Shultz told a Senate subcommittee whose members were complaining bitterly about civilian murders attributed to Salvadoran security forces, "In the end, if they don't clean up their act, the support in Congress is going to dry up. They know that. They've been told that."