Washington — Lawmakers returned to Capitol Hill after a five-week break of politicking and vacations with foreign policy on their minds. On the front row of the House chamber, about 20 representatives lined up to deliver their first one-minute speeches of the new session. All but a handful expressed outrage about the Soviet Union's downing of the South Korean airliner.
''A merciless act,'' a ''cold-blooded murder,'' and a ''horrifying reminder, '' said members. Rep. Edward J. Markey (D) of Massachusetts, the leader of the nuclear-weapons freeze, said he hoped the incident would not doom arms negotiations.
Leaders on both sides prepared to pass by midweek resolutions condemning the Soviet Union. And even House Speaker Thomas P. O'Neill Jr. hinted that President Reagan's hand may be stronger now.
The President's request for more money for the International Monetary Fund ''would have been gone and the MX would have been in deep, deep trouble'' last month, said the Massachusetts Democrat. He told reporters crammed into his chandeliered ceremonial office for a press conference that he would have to wait and see how strong the President will be as a result of the Korean crisis.
However, the speaker declined to criticize him for the mild sanctions imposed against the Soviets. Asked whether Mr. Reagan should have taken stronger action, Mr. O'Neill said, ''Like what?''
Senate majority leader Howard H. Baker Jr. (R) of Tennessee told reporters that ''I think (the Korean incident) would strengthen the President's position for a stronger military and in foreign policy.''
While resolutions on the South Korea airliner incident are only symbolic, clearly the most pressing issue on the Hill now is the presence of US marines in Lebanon. Mail from the public now is running overwhelmingly in favor of getting the troops home, Speaker O'Neill said Monday.
The administration and both parties attempting to iron out some arrangement that would satisfy jittery congressman and keep a united front on the Lebanese policy. ''My personal feeling is that we can't cut bait and run,'' said Speaker O'Neill. ''The people of the world would lose faith and confidence and credence in the government which is the leader of freedom in the world.''
''We want to work together,'' said the Democrat, who also said that the President should follow the War Powers Act, which requires consultation with Congress when sending troops into conflict.
As Congress reconvened, Democrats seemed wary of tying the President's hands on Lebanon. Minority leader Robert C. Byrd (D) of West Virginia said, as the Senate reopened at noon Monday, that it would be up to the GOP to challenge Reagan. ''Right now, the President would receive strong support from the Congress,'' he predicted.
Sen. Charles McC. Mathias Jr. (R) of Maryland is leading the way, so far. He introduced a resolution that would permit the US troops to stay at current levels in Lebanon for six months, but would require congressional approval for stepping up US involvement. His plan may be overtaken by a proposal that Senator Baker said he will work out with other Hill leaders and the White House.
Although foreign affairs dominated the opening day, Speaker O'Neill made a perhaps futile effort to shift the focus to domestic issues. He gave a long list of pending bills, including aid for the unemployed and the disabled, and defended adding $1.62 billion to 10 social programs cut by Reagan's 1981 budget.
''The American public appreciates the fact that the wrong thing was done'' said Mr. O'Neill, citing cuts in such social programs.
''We feel that there's a call to right the wrong and evils that were done,'' said the Speaker. Such proposals have little chance in the GOP-controlled Senate , but they could be an opening salvo in the coming election year.