Still dizzy from surprise events in two foreign countries, a divided and uncertain Congress is moving with caution. The only action that now seems definite is that the lawmakers will declare that the War Powers Resolution of 1973 took effect when United States marines landed in Grenada Oct. 25. The war powers law gives the President 60 to 90 days to remove the troops unless Congress authorizes them to stay longer.
But that move, which passed in the Senate Friday and is expected to be duplicated in the House on Tuesday, is hardly a bold stroke. ''I do not interpret this amendment as in any way a rebuke to the President,'' said Sen. Charles H. Percy (R) of Illinois, chairman of the Senate Foreign Relations Committee. The Senate was merely saying ''that we take the War Powers Resolution seriously,'' he said.
Since the widely held belief on Capitol Hill is that the fighting will be over soon on the tiny Caribbean island, the vote to invoke the law is largely symbolic now. Only if US troops are drawn into a protracted guerrilla war would the war powers law trigger a constitutional confrontation between Congress and the President.
The Senate also will this week consider sending a fact-finding mission to Grenada in response to the limited press access there.
Meanwhile, GOP lawmakers are lining up to praise the President for taking decisive action in Grenada, and Democrats are reluctant to criticize the expedition, which many say has widespread public support.
The public has ''a visceral instinct to support the Grenada action,'' says Rep. Bill Alexander (D) of Arkansas. ''How can you argue with these students who are coming in and kissing the ground?'' he said of the American medical students evacuated from Grenada.
''I'm more frightened about Lebanon,'' he says, adding that the US involvement there has a ''long-range potential.''
''We went into Grenada, and we won with the flags waving,'' says Rep. Tony Coelho (D) of California, chairman of the Democratic Congressional Campaign Committee. ''In the short term, the political gain is definitely positive'' for the President, he says, although he calls invading another country ''wrong.''
One of the few members who finds political problems in the Grenada invasion is Rep. Olympia J. Snowe (R) of Maine. A poll in her state registered 2 to 1 against it.
The first signs of serious opposition on Grenada came last Friday when House Speaker Thomas P. O'Neill Jr. (D) of Massachusetts, who had withheld criticism all week, finally spoke out. As the gunshots began to die down on the island, he fired a blast at President Reagan.
''His policy is wrong,'' said the speaker. ''His policy is frightening. Even though it was a success, it was a wrong policy,'' he said, charging the Reagan administration with practicing ''gunboat diplomacy.''
''If the lives of American citizens were in jeopardy, he had a right to go in ,'' the speaker said. But he said he ''questioned that'' and announced that the House Foreign Affairs Committee will investigate whether the Americans in Grenada were really in danger.
However, the speaker has continued to back US policy in Lebanon, despite the massacre Oct. 23 of more than 200 marines serving there and despite widespread opposition both in his party and in the public.
Instead of attacking the policy, Mr. O'Neill is aiming at whether the US is providing proper safety for the marines in Lebanon. The House Armed Services Committee launches an investigation next week by calling on the military and State Department to testify.
At the same time, the speaker is trying to defeat a move on Wednesday when opponents of keeping the troops in Lebanon are scheduled to bring up a motion to cut off funding. That vote is expected to be close.