Congress returns to face a stronger Ronald Reagan. Jetliner fallout will help MX, but may not extend to Lebanon

Congress returns to Capitol Hill after an explosion of international events that have upstaged most other issues. While the domestic front has been fairly quiet during the legislative recess, the shooting down of Korean Air Lines Flight 007 and the loss of four American marines in Lebanon have set the tone for the new congressional session.

''It is fair to say that the President's hand has been strengthened on both things of a defense and of a foreign-policy nature,'' a Senate leadership aide says. Mr. Reagan's moderate response after a Soviet jet destroyed the Korean passenger plane ''helped deflate the criticism that he's a warmonger,'' he adds.

But the aide says it would be a mistake to lump all foreign issues together, especially because of growing concern over US peacekeeping forces in Lebanon. ''Congress wants to have a little more say-so in what's going on,'' he says.

Senate majority leader Howard H. Baker Jr. has never been enthusiastic about sending troops to Lebanon. The Tennessee Republican opposed the idea when it was first floated more than a year ago.

On the House side, Rep. Clarence D. Long (D) of Maryland is seeking to cut off funds for the troops in Lebanon unless the President files a report under the War Powers Act justifying their presence. Congressman Long, chairman of the House appropriations subcommittee on foreign operations, calls the US troops ''sitting-duck targets in an undeclared war.'' He says he will bring up his proposal the second week after Congress returns.

However, Long said last week that he was moving without the go-ahead from the Democratic House leadership. His proposal may face tough going in the face of President Reagan's enhanced position since the Korean airliner tragedy.

Defending the Reagan policy during the recess was House minority leader Robert H. Michel, who told a group of his constituents in Peoria, Ill., ''We've got to do what we can to stabilize that situation in Lebanon.''

Despite the loss of American lives, the GOP leader opposed pulling out US troops. ''Civilian casualties would be in the hundreds of thousands'' if the marines left, he warned.

Meanwhile, questions still linger about Central America, although controversy over US policy there has subsided in recent weeks. The next three weeks will test whether the new Reagan clout stretches to that region.

As of Oct. 1, two legal restraints on Reagan's Central America policy will expire. These include the Boland amendment, which outlaws US aid to overthrow the government of Nicaragua, and the congressional requirement that the administration certify that there has been progress on human rights in El Salvador every six months. Unless Congress acts, those limits will be removed.

Congress will also have to determine how much aid it will send El Salvador during the new fiscal year that begins Oct. 1. ''How much money will be very controversial,'' says Rep. Michael D. Barnes (D) of Maryland, chairman of the House subcommittee on Latin America.

''I think there will be some spillover effect from the Korean airliner downing,'' he says. ''The intensity of feeling is so strong that it's bound to affect other issues.''

But he says that Congress will not cut all the strings on aid to Central America and predicts that it will set some conditions for El Salvador aid. However, the Boland amendment could be in trouble, especially in the Senate, according to Representative Barnes.

On defense, Reagan is predicted to win an easy victory for building the MX missile soon after Congress reconvenes.

And he could win a backdoor victory in another area. The 1984 budget that Congress passed over his objections is heading for almost certain oblivion. Although both houses have passed the budget in its general outline, prospects for enforcing it look dim.

The biggest sticking point is the $73 billion in new taxes called for over the next three years. The White House opposes the tax hike, and no congressional leader has yet taken up the thankless task of pushing for a tax increase.

So, for the first time since Congress imposed the discipline of a budget process on itself in 1974, it is returning to its old budget-less ways. During the next weeks Congress will write spending bills to keep the government operating after Oct. 1. The club poised over those spending bills is Reagan's veto.

Also on the congressional plate:

Martin Luther King Jr. holiday. Already passed in the House, it has enough Republican and conservative supporters to make quick Senate approval likely.

Health insurance for the unemployed. The House has passed a $4 billion version, and the Senate is considering a $1.8 billion plan.But momentum has slowed, and action will be delayed, a Senate leadership aide says.

Immigration reform. A sweeping bill to reduce the flow of illegal aliens while offering legal status to many now living in the United States has passed in the Senate and awaits House action.

Natural gas deregulation. Hotly contested and conflicting proposals are moving through House and Senate. The issue is so contentious that it is not clear whether either house can complete action.

Coal slurry pipelines. As one of its first bills, the House will consider a proposal that would clear the way to build thousands of miles of pipelines to transport pulverized coal. Farmers concerned about the amount of water the system would use and railroads upset over losing their coal-hauling business are objecting.