The 100th Congress returns Monday for an election-year session dominated by issues of war, peace and politics as Secretary of State George P. Shultz and Defense Secretary Frank C. Carlucci join Senate leaders in formally launching the drive for ratification of the new U.S.-Soviet nuclear arms treaty.

Conservative opposition has dwindled, and Senate leaders are cautiously predicting approval of the Intermediate-Range Nuclear Forces (INF) Treaty by early spring -- presumably in time for President Reagan's summit meeting in Moscow with Soviet leader Mikhail Gorbachev, tentatively planned for May.

But final approval is not expected before an intense congressional struggle, fraught with opportunities for crippling treaty restrictions and moves aimed at derailing an even more controversial strategic arms reduction treaty (START) under negotiation to reduce long-range strategic nuclear forces.

For many in Congress, the INF struggle almost certainly will help frame a broader debate on critical issues involving both conventional and nuclear military strategy, the future of NATO and further arms agreements with the Soviets -- both for the presidential election campaign and for the new administration it will produce.

This struggle will unfold in a month or more of hearings by three committees, starting with simultaneous appearances Monday morning of Shultz before the Senate Foreign Relations Committee and Carlucci, accompanied by Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, before the Senate Armed Services Committee. The Senate Select Committee on Intelligence is expected to begin its closed-door study of verification procedures later in the week.

The Foreign Relations Committee plans to assemble its findings and those of the other two panels by late February and report the treaty and its resolution of ratification, along with proposed amendments, reservations and other provisos, to the Senate floor in early March. Senate leaders expect at least a month of floor debate before a final vote on the ratification resolution, which needs a two-thirds vote for approval.

Witnesses at the hearings will range from soldiers to scholars, reflecting a wide span of views on the treaty, and include most living former secretaries of state and defense, going back to Clark M. Clifford, defense secretary in the Johnson administration.

But politics of the present will also intrude. Senate Republican leader Robert J. Dole (Kan.), who rarely misses a spotlight in his campaign for the GOP presidential nomination, will appear as "guest" of the Foreign Relations Committee to testify on the treaty's behalf even before Shultz is heard.

This is the first time the Senate has considered an arms-control treaty since it debated the Strategic Arms Limitation Treaty (SALT II) in 1979 but did not vote on it. That treaty and several others with the Soviets remain unratified.

In contrast with the partisan paralysis that has marked relations between the White House and Congress over the past seven years, the treaty debate will underscore the broad bipartisan support for the pact, which Reagan signed with Gorbachev last month to eliminate medium- and shorter-range nuclear missiles.

Congressional leaders will be looking to Reagan's State of the Union address Monday night for clues to whether this collective approach will spread to other issues, such as trade and a variety of Democratic social welfare initiatives that prompted 1987 veto threats.

Expected Senate confirmation of Supreme Court nominee Anthony M. Kennedy next month, following failure of Reagan's two previous choices, could ease frictions. Also, barring economic reversals, a two-year deficit-reduction agreement achieved late last year could make the budget less volatile this year. Congressional leaders have vowed to pass appropriations bills on time to avoid repeating last year's tumultuous efforts to fund the government in one huge bill at the last minute.

But a Feb. 3-4 showdown in the House and Senate over Reagan's expected request for more aid for the Nicaraguan contras is considered virtually certain to reopen one of the most sensitive wounds in the president's relations with Congress. The vote could be close, with the outcome hinging on peace efforts by Central American leaders and the packaging of Reagan's aid proposal, according to lawmakers.

Reagan will be seeking in his final year to preserve his legacy while the Democrats will be protecting their congressional base and pushing to recapture the White House -- a highly combustible mix for a session that will be sharply abbreviated by political campaigns and conventions.

On the INF Treaty, Democratic leaders are firmly allied with the president and have banded together with Senate Republican leaders to fend off amendments that could force renegotiation of the pact.

Despite a mobilization of conservative groups outside Congress to build up pressure against the treaty, opposition within the Senate appears to have faded. Some leaders are predicting that it will pass with 90 or more votes; a two-thirds majority is required.

But lingering controversy over verification and compliance, the treaty's impact on the balance of conventional military forces in Europe and its implications for future reductions of long-range weapons could delay approval or possibly even jeopardize the pact in the form of deadly amendments or reservations, treaty proponents have warned. Such provisos can be cloaked in politically appealing language and approved by majority vote, making them easier to pass than the treaty itself.

Sen. Jesse Helms (N.C.), ranking Republican on the Foreign Relations Committee and a leading critic of the treaty, has signaled his intention to seek an amendment addressing what he called "significant defects and loopholes," including the fact that warheads can be salvaged and used on other weapons. Late Friday Helms released to GOP members of the Foreign Relations Committee a 180-page memorandum that attacked the treaty on a variety of counts and said it would benefit the Soviets at the expense of the United States.

Other conservatives are suggesting proposals that would tie withdrawal of U.S. missiles to Soviet compliance with existing treaties as well as the INF pact, along with other moves that could cause the Soviets to balk at final ratification. But few have said publicly they will vote against the treaty or support amendments that could scuttle it.

More serious than these objections, some say, is the apparently mounting Senate concern over the effect that elimination of medium-range nuclear weapons will have on the balance of conventional military forces between NATO and the Soviet-led Warsaw Pact, on relations within NATO and on budget-strained defense resources here.

A recent report concluding that conventional forces are already in "uneasy" balance, prepared by Sen. Carl Levin (D-Mich.), chairman of the Armed Services subcommittee on conventional forces, was seen as a bid to temper debate on the issue.

But concern on this issue stretches beyond the far right of the Republicans in the Senate. Dole and Senate Armed Services Committee Chairman Sam Nunn (D-Ga.) have voiced concern about the implications of the INF pact on conventional-force balance. Nunn has talked about using the treaty as leverage to improve the relative strength of NATO forces through a combination of allied force modernization and negotiations to reach a conventional-force reduction agreement with the Soviets. He has not ruled out his suggestion of last year to tie the final phase of INF withdrawals to movement on conventional forces, and a move along these lines could attract considerable support.

But Republican as well as Democratic strategists expect all these reservations from the broad center of the Senate's political spectrum to stop short of language that could scuttle the INF pact, focusing instead on influencing U.S. and NATO actions and Reagan's bargaining position for the START talks.

Sen. Alan Cranston (D-Calif.), a leading pro-INF strategist and chief nose-counter for the Democratic majority, has said he believes there is a majority to reject all crippling amendments as well as the 67 votes needed to approve a resolution of ratification for the treaty, which would clear the way for the final exchange of ratification papers by Reagan and Gorbachev.

Just to make sure, Cranston and other pro-INF senators are working on a plan that would give senators a chance to vote on a half-dozen or more key issues, including Soviet withdrawal from Afghanistan as well as INF-related problems, in a manner that would not require assent from the Soviets.

Although Senate Foreign Relations Committee Chairman Claiborne Pell (D-R.I) will manage the treaty in committee and on the floor, key roles also will be played by an impressive array of other Senate leaders who do not often work in harness. They include Senate Majority Leader Robert C. Byrd (D-W.Va.), Dole, Cranston, Nunn and Sen. Ted Stevens (R-Alaska) as Pell's Republican counterpart in leadership of the Senate's negotiations observer group. The intelligence committee, headed by Sens. David L. Boren (D-Okla.) and William S. Cohen (R-Maine), will be a key forum, since any hitches in the treaty's unprecedented verification safeguards could give its foes an opening for attack that they do not now seem to have. "Unless there is something wrong with the verification procedures, I don't foresee anything that would endanger this treaty," Byrd said in an interview.

On other issues, congressional leaders are pushing for House-Senate conference agreements in the first few months of the session on legislation to toughen trade rules and a new catastrophic health care program for Medicare, along with Senate action on welfare reform.

Congressional action is needed to implement the newly negotiated trade agreement with Canada, and another push will be made to pass an extension of the Clean Air Act. Byrd has indicated he will try again to win approval for campaign-financing limits for congressional elections. Most of organized labor's agenda is pending, including legislation to raise the minimum wage for the first time in seven years.