The US Congress is feeling the full impact of the activist Reagan presidency in the space of a few days. In times past, lawmakers have frequently intoned that Presidents merely propose, while Congress must ''dispose.'' This week they found that President Reagan acts, leaving little for them to do except react.
Legislators, accustomed to the painstakingly slow pace of passing laws, are reeling from events about which they can only ask questions, peppered with protests. They went from private briefings with the secretaries of State and Defense on the killings of more than 200 marines in Lebanon to more briefings following the news that Mr. Reagan had sent troops into Grenada.
And in the midst of the crises abroad, the President launched a surprise assault on the domestic front. He effectively dismantled the United States Civil Rights Commission, which has been critical of his civil rights record, by firing three of its members. This leaves only three incumbents, not enough for a legal quorum.
By midweek, some on Capitol Hill were almost punchy from news shocks. Reagan had the power to dismiss the commissioners, conceded Sen. Joseph R. Biden Jr. (D) of Delaware, but he added, ''I can't imagine how he found time to think about it.
''Between wars, I haven't been able to figure out who's even here,'' said Senator Biden, when asked if he had enough votes to establish a new independent panel to monitor civil rights progress.
''It's very frustrating for the public to sort through all of this,'' said Sen. Nancy Landon Kassebaum (R) of Kansas, a member of the Senate Foreign Relations Committee. She told a breakfast meeting of reporters Wednesday that she was ''troubled'' by both the timing and the justification of the invasion of the east Caribbean island of Grenada.
''I wondered why we decided to do this on the day that we did, why we couldn't have put it off,'' she said of the invasion, charging that it is diverting the administration's attention from finding a diplomatic solution in Lebanon.
She said that while earlier briefings on Lebanon had been adequate, ''yesterday's briefing (on Grenada), I think, left people more troubled than before.''
Along with other Democratic and Republican critics, she said that the administration had failed to provide convincing evidence that American students on the island had sought aid from the United States.
Senate majority leader Howard H. Baker Jr. (R) of Tennessee contended that he was satisfied with the administration's explanation.
''Baker was pleased with the way the briefings went,'' a spokesman said. ''You would expect some disagreement.''
The deepest disagreement is among House Democrats, whose leadership has staunchly defended US policy in Lebanon and just as staunchly refused to criticize the Grenada expedition until it ends.
House Speaker Thomas P. (Tip) O'Neill Jr. of Massachusetts walked into an emotional Democratic Caucus meeting, called by members opposed to the Lebanon policy, and gave a ringing speech.
''He was as tough as I've ever seen him, articulating that this is not a partisan issue,'' says Rep. Tony Coelho (D) of California. ''There's a lot of frustration that people want to get out. But I think Tip was saying basically, 'I share your frustration, but we've got to be responsible.' ''
For now Congress has few means to counter the enormous clout wielded by the President. Both critics and supporters are predicting that opposition to the Grenada action will be short lived, since the troops will probably by pulled out soon anyway.
Opponents of keeping American marines in the peacekeeping force in Lebanon will have to wait until at least next week to move in the House.
Speaker O'Neill has delayed until then a defense appropriations bill that contains a proposed amendment that would cut off funds for the military forces in Lebanon. By then it is expected that feelings will have cooled somewhat.
It is symbolic that the only return punch Capitol Hill has delivered was in overriding a Reagan veto of an obscure bill conveying 3.11 acres of federal land in Oregon to six couples and two companies.