Crises abroad and politics at home will top a crowded agenda of national security and domestic issues as Congress returns to town Monday from its five-week summer interlude of politics, play and junkets.

A key question is whether Congress and President Reagan, who achieved a kind of wary coexistence during the first half of the year, will be drawn together or divided as an election year approaches during a period of international stress.

At least at the start, the Soviet Union's downing of a South Korean jetliner and the deaths of U.S. Marines in Lebanon are considered likely to deflect Congress' attention from its customary preoccupation with money matters as the new fiscal year, which begins Oct. 1, draws near.

But in addition to spending bills, a debt ceiling extension and several authorization bills that must be passed by the end of the month, a variety of politically laden domestic issues are high on the list for early action, especially in the Democratic-controlled House. Republicans dourly refer to them as "the speaker's political agenda" for 1984.

The Republican majority in the Senate, nervous over whether it will retain control after the 1984 elections, will also increasingly have its eye on the shortest road to political survival.

Republicans suffered a blow during the recess when Armed Services Committee Chairman John G. Tower (R-Tex.) joined Majority Leader Howard H. Baker Jr. (R-Tenn.) in becoming a lame duck who is not seeking reelection next year. There is speculation that one or more others may do the same, especially if the alternative is coming back as a member of a powerless minority.

But the GOP at least temporarily gained a seat, giving it a 55-to-45 majority, with the appointment of former Washington state governor Daniel J. Evans (R) to succeed the late Sen. Henry M. Jackson (D-Wash.), who died during the recess.

Although Jackson's death deprives Congress of one of its most forceful and respected champions of defense and critics of the Soviet Union, the outraged response to the downing of Korean Air Lines Flight 007 could rally the previously restive Congress behind the president on national security issues, some leading figures in Congress have said. The question is how much and how long.

Swift action is expected in both houses on a resolution condemning the Soviet action in shooting down the jetliner.

There also may be a push from within Congress for speedy congressional authorization for the U.S. Marines to remain in Lebanon, as many lawmakers say is required under the War Powers Act so long as hostilities continue there. But some influential members want the marines brought home, and the authorization could prove controversial, inviting restrictions even if it is approved.

Both houses are expected to act this week on a $187.5 billion defense authorization compromise that gives a go-ahead for production of the MX missiles, a new type of nerve gas weapon and most other weaponry sought by Reagan. Support for the House-Senate conference agreement was considered shaky in the House, although it may have been shored up if members perceive the vote as a way of demonstrating a strong congressional response to the Soviets.

The House also is expected to deal this week with two major Democratic initiatives aimed at reversing some of Reagan's earlier cost-cutting and at helping to define the Democrats' position for next year's elections.

One would raise authorization levels by $1.6 billion for 10 major education and social welfare programs, ranging from educational aid to disadvantaged children to nutritional assistance for pregnant women, in order to restore spending authority cut two years ago at Reagan's behest.

Generally, the new levels would permit higher spending sanctioned by the fiscal 1984 budget that Congress approved last spring over Reagan's protests. Republicans have warned that Reagan would veto the authorization increase.

The other bill would pump new life into the trade adjustment assistance program for workers and businesses that have been hit hard by imports. The administration wants to terminate the program when it expires Sept. 30, but the Democrats want to extend it for two more years and greatly expand benefits, which were cut drastically in the Reagan administration's first year.

Lined up just behind these measures are two major Democratic jobs bills: one to authorize $3.2 billion for local government grants for public works jobs and another to authorize $5 billion for local public service jobs. Like most of the House Democrats' recent jobs initiatives, neither is expected to go far in the Senate.

The filibuster-plagued Senate will pick up pretty much where it left off: with a talkathon against Reagan's proposal to create Radio Marti for broadcasts to Cuba. Another pending matter, the Interior Department appropriations bill, is also bogged down in a filibuster.

For the rest of the month, the major business of both houses will be a scramble to pass as many as possible of the 13 regular appropriations bills for fiscal 1984 beginning Oct. 1. As usual, the rest--probably at least half of the total--will have to be wrapped into an omnibus stopgap "continuing resolution" to keep the government operating after current funding runs out Sept. 30.

Work on the huge defense appropriations bill, including funding for the MX and other controversial weapons, is scheduled to begin this week in the House and shortly thereafter in the Senate. But Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House Appropriations subcommittee on defense, has indicated that he may dawdle over the bill if it appears that the emotional fallout from the Korean air disaster is leading to spending excesses.

In any case, defense, foreign aid and the big social welfare spending bill (labor, health and human services) are expected to be put in the stopgap resolution, at least until regular appropriations for these activities are approved. Only four, mostly minor, appropriations bills have been passed and signed by the president so far.

Controversial features in pending appropriations bills range from a proposed moratorium on Interior Secretary James G. Watt's coal leasing practices to abortion funding for federal workers, as well as overall spending levels that Reagan considers excessive in some of the bills.

Leading the list of duties most likely to be left undone is the congressional budget's mandate for $85.3 billion in deficit reductions over the next three years. Although Senate and House committees appear to be having little trouble meeting the requirement for $12.3 billion in spending cuts, mostly from Medicare and pension programs, Republicans and Democrats alike have balked at $73 billion in tax increases, starting with $12 billion next year.

The deadline for action on this so-called budget "reconciliation," already postponed once, is Sept. 23, and neither tax-writing committee has shown any hurry to meet it.

But the prospect of deficits nudging $200 billion for the foreseeable future, possibly forcing another surge in interest rates, is prompting sporadic cries for a budget "summit" or a high-level bipartisan commission that would give everyone a political cover for raising taxes. There are also proposals for linking spending cuts to tax increases, but chances for more than token action are considered dim until after the 1984 elections.

Congress will have one unavoidable encounter with the deficit this month because the Senate must vote to raise the federal debt ceiling when existing borrowing authority runs out, probably about Oct. 1. The House already has voted to raise the ceiling from $1.389 trillion to $1.614 trillion, presumably enough to last through the 1984 fiscal year, but the Senate has not yet acted.

Because the continuing resolution and debt ceiling legislation are "must" items with fixed deadlines, either or both may become targets for controversial riders that could lead to nasty confrontations in Congress or between Congress and Reagan.

Passage of the intelligence authorization bill is also required this month, raising the prospect of another fight over ending covert aid to forces fighting the leftist regime in Nicaragua. The House approved such a ban earlier this year but the Senate refused to go along.

The Senate Foreign Relations Committee is scheduled to consider later this month a nuclear freeze resolution, which passed the House earlier in the year in modified form. But the pro-freeze lobby is fearful of an emotional backlash if action comes too soon after the Soviet attack on the airliner. Prospects for Senate approval were considered bleak even before the attack.

Another struggle over an administration-sponsored $8.4 billion increase in the U.S. contribution to the International Monetary Fund is also anticipated. The measure, awaiting a Senate-House conference, faces trouble when it goes back to the House because of Democratic anger over a GOP campaign tract that criticizes Democrats who supported the measure.

House Banking Committee Chairman Fernand J. St Germain (D-R.I.) has warned that the measure may fail unless the administration persuades the Senate to pass the big housing authorization bill that he wants, which would continue subsidized housing programs that Reagan wants to kill.

The popular revenue-sharing program for local governments expires this month and must be reauthorized; the House has approved its version and action is pending in the Senate.

A similar deadline faces the Export Administration Act, which governs the president's powers to control exports for foreign policy purposes.

The Senate will take up, and is expected to approve, House-passed legislation to make the birthday of the late Rev. Dr. Martin Luther King Jr. a national holiday.

Also on the agenda are several farm bills, including a freeze on target prices for grains and cotton and an overhaul of costly dairy price supports.

Other proposals before Congress would revise immigration laws, deregulate natural gas, provide for hazardous waste disposal, update the Clean Water Act, extend supplemental unemployment benefits into fiscal 1984, slow the purging of disability benefit rolls, protect the auto industry from import competition, reauthorize the Legal Services Corp. and the U.S. Civil Rights Commission, authorize $12.4 billion in new water projects, create new bankruptcy courts and facilitate construction of coal slurry pipelines.