Talking power with the top Democrat in Washington

House Speaker Thomas P. O'Neill Jr. sat back in his swivel chair and watched the television report on Grenada in silence. Earlier in the week he had been summoned to the White House for a top-secret meeting and told to arrive at a side entrance. On the way, he recalled telling an aide, ''I bet they've invaded Grenada.'' When he learned his guess was right, he made clear that the White House informed him, not consulted with him before sending the Marines.

But while the fighting continued, the Massachusetts Democrat is taking care not to let a critical word slip. ''I don't want the Marines to say, 'Hey, they're criticizing us at home,' '' Speaker O'Neill said in an interview. But he added, ''I'll have a few words to say about Grenada'' later.

For the white-maned Democrat, who has stood his liberal ground for two years and led the opposition to Ronald Reagan, these have been awkward times. Not only has he stored any harsh words on Grenada, but he has been the President's most important ally on keeping US marines in the peacekeeping forces of Lebanon. O'Neill threw his weight behind the Reagan policy, helping to pass an agreement to permit the marines to stay for up to 18 months.

''I think (the President) would have suffered a defeat'' otherwise, said O'Neill. ''I have a good, loyal group within my own party that will really fight for me.''

Since the news of the loss of American lives, Democratic frustration about the Lebanon policy has risen. Critics called a meeting of the Democratic Caucus Wednesday, and participants in the closed-door session called it highly emotional. If the majority of Democrats opposed the 18-month plan earlier, it became clear that more have doubts now.

''There's a judgment in their minds whether I'm right or whether I'm wrong on this thing,'' said O'Neill of his fellow Democrats. But asked if he sees the disagreement as a challenge to his leadership, he said, ''The answer is no.''

''I'm doing what I think is best for the country first, and for the party,'' he said, pointing out that he was the one who ''stayed the course'' for the Democrats despite criticism and defections during the early Reagan years.

''I think there's a certain admiration out there for the fact that I am giving leadership,'' said the speaker.

He displayed that leadership during a stirring Democratic Caucus speech, asking members to oppose cutting off funds to the troops in Lebanon. Some listeners called it the best speech of his career, and it brought even his critics to their feet, applauding the man, if not the policy.

''He's been the point man for Democratic criticism of this administration'' on issues ranging from the MX missile to Central America, said House Democratic whip Thomas S. Foley (D) of Washington later. But he added of the speaker, ''He does believe that where national interests are involved, he should put politics aside.''

Nonetheless, O'Neill is putting distance between himself and the Reagan administration on Lebanon.

''There's no way we can have them there in a suicidal swamp,'' he said of the position of the marines at the Beirut airport. He blamed a ''military blunder'' for the mass killings, saying that the guards should have had bazookas or antitank weapons to destroy any bomb-laden truck.

And he complained aloud that the ''simplified'' explanation that persuaded him to go along with the Reagan administration ''hasn't turned out to be that simplified.''

Setting aside troublesome foreign policy questions, O'Neill has registered a fair share of victories in recent months. Shortly before the explosion in Beirut , he was surveying the domestic political scene with an unmistakable air of I-told-you-so.

''I always anticipated times would change,'' he said. Except for passage of the MX missile, ''the President hasn't had a single victory in a year and a half.

''Everything else that we've passed here we . . . passed with Democratic votes and some moderate Republicans, sent it to the Senate and compromised it out,'' he said, and then Republican congressional leaders had ''to get the President to sign it.'' Speaker O'Neill listed as examples bills ranging from the 1982 tax hike to social security reform and jobs and job-training legislation.

Republicans who once ridiculed O'Neill have grown quieter. Democratic ''boll weevil'' conservatives who opposed him have been chastised by both voters and party and almost shrunk out of sight.

Even President Reagan has discovered that he sometimes must go to O'Neill if he wants legislation. On the other hand, when O'Neill takes a stand against the administration, he can sometimes win by refusing to act.

''There's no question that I have power. I have the power to schedule,'' he said. ''That's the most important power that the speaker has. I can put to the floor what legislation I want to go to the floor.''

While O'Neill can halt some Reagan-backed proposals, the speaker has seen some of his own bills stopped as well. The Democratic House has passed jobs, housing, and other relief legislation, as well as a nuclear weapons freeze, which have been ignored or rejected in the Senate.

He also has not been able to halt US aid to rebels fighting the Sandinista regime in Nicaragua, where O'Neill has said that the US is ''opposing a legitimate government.'' Although the House has voted twice to cut off American aid to the rebels, the aid continues to flow because the Senate has not acted.

On the US economy, O'Neill said it is improving, but ''it hasn't happened because of anything that (President Reagan) did. It's the strength of this nation.

''I don't want to appear as one who believes in gloom or doom or anything like that,'' he continued, but he predicted that with $200 billion federal deficits, 10 million people unemployed, and $100 billion in international trade deficits, the upswing in the economy will not last. ''I hate to say it, but I think it's temporary,'' he said.