Republicans of all stripes and many Democrats said last night they believed President Reagan had buttressed support for his actions in Lebanon and Grenada by his televised speech to the nation.

From Richard Viguerie, the direct-mail publicist of conservative causes, to Rep. Paul Simon of Illinois, a liberal Democratic Senate aspirant, there was praise for the Reagan address.

Many of those who commented cautioned that there were still important doubts left unresolved. But for the most part, the response suggested that the combination of Reagan's emotional evocation of the Marines' gallantry in Lebanon and the description he gave of a security threat from the Cuban forces on Grenada impressed fellow politicians as weighty.

"I think he made an effective presentation," said Sen. Lloyd Bentsen of Texas, the chairman of the Democratic Senatorial Campaign Committee. "I believe what he did in Grenada will be a plus for him, but Lebanon's a different case."

Rep. Claudine Schneider of Rhode Island, a GOP moderate who is weighing a Senate race, said the comments from her constituents since Sunday's terrorist bombing of the Marine headquarters in Beirut caused more than 220 deaths "had been running 10 to 1 in favor of bringing the boys home." But Schneider, who had not endorsed a precipitous pullout herself, said the Reagan speech "could turn that around."

The initial reaction indicated that in the eyes of many politicians, Reagan had stepped up to the largest foreign policy crisis of his presidency with one of his better oratorical efforts. If that response is borne out in coming days, he could also have buttressed his chances for reelection.

There were critics, however. Rep. Samuel S. Stratton (D-N.Y.), a normally hawkish member of the House Armed Services Committee, said Reagan "talked about bringing stability to Lebanon, but the Marine force has not brought stability. They have just been sitting there. They're a target."

Bill Hamilton, a Democratic pollster whose clients include presidential aspirant John Glenn, the senator from Ohio, said the Reagan speech sounded "extremely defensive" to him. Hamilton predicted that despite the address, "the fear of war" that has been part of Reagan's "gender gap" problem will be "locked in" by the events in Lebanon and Grenada.

However, the range of endorsements for Reagan's rationale was impressive. Viguerie, who condemned Reagan's reaction to the Soviet attack on the South Korean airliner as woefully inadequate, sent the president a telegram last night saying "the light of freedom shines brighter in the world . . . because of your decisive leadership in overthrowing the communists in Grenada."

Across the political spectrum, Simon, a supporter of Sen. Edward M. Kennedy's 1980 bid for the Democratic presidential nomination, said "one of the president's most eloquent addresses" would likely strengthen his position on both Grenada and Lebanon.

In a statement released by the White House, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) called it a "powerful and moving speech, stating clearly a policy of resolve and concern." Added Baker: "Undoubtedly it will have a unifying effect on the American people."

But Sen. Sam Nunn (D-Ga.), of the Armed Services Committee, dissented on Lebanon. While saying he was "grateful" that the mission in Grenada was "well-defined" and backed by strong military force, he said in regard to Lebanon: "If Lebanon is vital to our national interests, as the president described, we do not have enough military force to protect this interest or even enough military forces to protect themselves."

Sen. Nancy Landon Kassebaum (R-Kan.) said Reagan was "particularly eloquent" on Lebanon and made "a good case for preventive action" in Grenada. "My doubts are really questions in the history of it all . . . , " she added.

Comparing the address to one of President Roosevelt's fireside chats, Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) said Reagan "took a highly complex situation and boiled it down to its essentials."

Sen. Claiborne Pell (D-R.I.), ranking Democrat on the committee, said Reagan gave an "excellent presentation of the administration's viewpoint" but, in regard to the safety of U.S. students in Grenada, "the danger came more with the invasion than before."

Sen. Thomas F. Eagleton (D-Mo.) said he agreed with Reagan that American troops are acting with courage in Lebanon but "I do not accept the purpose for which they are there."

Sen. Christopher J. Dodd (D-Conn.) said Reagan failed to make a convincing case for either the Lebanon or Grenada actions. "I think the verdict is still out. Either we have a good reason for being there, or we don't."

Sen. Larry Pressler (R-S.D.) praised the speech but said he thought the United States should have "a criterion for our troops withdrawing from Grenada and Lebanon."

"My wife and I couldn't help but think about President Kennedy at the time of the missile crisis," said Senate Majority Whip Ted Stevens (R-Alaska).

Bentsen said the Soviets "have a major influence" in both areas but the problem for the United States is not the same in each.

"In Grenada, it's a situation that we can correct with a minimum of loss," he said. "It can be quickly done and surgically done."

But the Texas senator argued that despite Soviet influence in the Middle East, "We ought to look for the first face-saving opportunity to substitute U.N. troops for our own . . . . I opposed going into Lebanon in the first place. It's a lot tougher to extricate ourselves now, and it can't be done quickly."

Gov. Richard Thornburgh (R-Pa.) said he believed Reagan had begun to define the mission of the Marines in Lebanon by emphasizing their importance in helping the diplomatic process go forward.

"Many people found the absence of that definition troubling," he said. "My shorthand is that the people of Pennsylvania were reassured."

Former South Dakota senator George McGovern, now a candidate for the Democratic presidential nomination, issued a short statement that did not directly touch on Reagan's speech. "I have been heavy-hearted all week because of the realization that if we had withdrawn our Marines during the cease-fire, as I advised repeatedly, over 200 young American Marines would be alive today," he said.

Frank J. Fahrenkopf Jr., Republican national chairman, said in a statement issued before the speech was televised, "The president has stated the case clearly . . . . The time for partisan bickering is in the past. Today, let us begin to pull together in common cause."

One Republican who watched the speech with 30 to 40 people outside of Washington said Reagan's emotionalism had been effective, even among those in the group who did not appear to be his supporters.

"I just had a very smart, professional woman say to me, 'That was one of the most emotional speeches I've ever heard,' " he said, asking not to be identified. "She ain't no bleeding heart."

"If the truth be known," he added, "I'll bet that Reagan wrote a lot of that speech. That was very much a Reagan speech."

But one Democrat, asking not to be identified, said, "I've never thought of Reagan more as an actor than tonight. Maybe the public will see some of that."

Members of the House, who have a professional's esteem for a successful political speech, said they were uniformly impressed by the power of the president's performance.

"It was a masterful speech," said Rep. Jim Leach (R-Iowa), in a comment echoed by many. "He told a grieving nation exactly what it wanted to hear."

On the substance of the address, however, the representatives' unanimity dissolved. For the most part, those who opposed Reagan's policy in Lebanon said he had failed to explicate the American role there; those who support the president said he explained the issue well.

"The president had a very fine blend, which was more peace through law than peace through strength," said Schneider, the Rhode Island Republican who has supported Reagan on Lebanon despite some misgivings. "There was also a greater emphasis on working with our allies, and that is significant."

Several Democrats, however, said the president's appeal was a disreputable effort to gloss over a failed policy with tales of military heroism.

"I don't think that invoking the death of brave young Americans is a reasoned justification for lack of a policy," said Rep. Ted Weiss (D-N.Y.).

The House members--even those supporting the president--were not certain that his speech could overcome what they see as a general sentiment in the country to get the American troops out of Beirut.

"The public generally would like us to get out of Lebanon," said Rep. Henry J. Hyde (R-Ill.). "I kind of doubt this speech will change that. It might help a little. But our presence in Lebanon is just not a popular policy in the country."

House members split in a similar way in regard to the president's remarks on Grenada. Those who support the invasion felt Reagan made a good case.

"I agree with him, and I think he put it correctly," Stratton said.

But Rep. Clement J. Zablocki (D-Wis.), chairman of the Foreign Affairs Committee, said the president "left many questions unanswered."

"I do not think the case has been convincingly made that there was danger to U.S. lives in Grenada," said Rep. Stephen J. Solarz (D-N.Y.).