The liberal Democratic speaker of the House yesterday offered another fiery defense of the conservative Republican president's military policy in Lebanon, but other Democrats in Congress continued to search for a way to bring the U.S. Marines home from Beirut.

Most congressional Democrats, meanwhile, continued to bite their tongues on the Grenada invasion, partly out of lack of knowledge about what is happening there and partly out of reluctance to criticize while the guns were still firing. But the Democrats made it clear they will have a great deal of criticism to offer when the Grenadan situation is settled.

In an rip-snorting speech to fellow Democrats at a closed caucus on the Lebanon question, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) roared that his party should put "patriotism above partisanship" and back President Reagan, according to members who were present.

At the caucus, as Democrats related it afterward, Reps. Samuel S. Stratton (D-N.Y.) and Clarence D. Long (D-Md.) argued that the president has failed to set forth what the Marines are supposed to achieve in Lebanon, and that without a clear mission they can be nothing more than stationary targets for terrorists. Several junior Democrats chimed in, saying their party was obliged to stand up to the Republican president on the issue.

By tradition, the last speech is reserved for the speaker, and O'Neill took the occasion to shout his contempt for Democrats who "try to make this a partisan issue." He is in agreement with Reagan, O'Neill said, "because I am a PATRIOT!"

His effort was rewarded, members said, with a standing ovation.

Those present said that O'Neill, face red with emotion and voice pitched at top volume, defended his agreement with President Reagan permitting the Marines to stay in Lebanon until March, 1985, and denounced legislation proposed by other Democrats to bring the troops home next month.

"It was the most emotional speech of his career," said a veteran O'Neill watcher on the House leadership staff.

On the U.S.-led invasion of Grenada that began Tuesday morning, Democratic presidential candidate and former vice president Walter F. Mondale said Reagan's explanation for sending in the troops was "unpersuasive" and that there is "considerable dispute" over whether Americans living on the Marxist, eastern Caribbean island were in any danger.

"If they were, in fact, not in danger, the key element in the administration's case is undermined," Mondale said at a news conference in St. Paul.

The invasion also undermines "the high ground this nation needs as a leader in the world in discouraging other nations, such as the Soviet Union, from doing what they are doing in Afghanistan, Poland, Cambodia and elsewhere," Mondale said.

For the time being, however, Capitol Hill Democrats generally contented themselves with jests and barbs on the Caribbean situation.

"We'd like to have another country to invade, but they can't find one small enough," said Sen. Patrick J. Leahy (D-Vt.). Rep. James M. Shannon (D-Mass.) invoked Cole Porter: "Tomato, tomahto, Grenada, Grenahda, let's call the whole thing off."

Senate Democrats caucused but were unable to come up with a consensus on the wisdom or propriety of the U.S. invasion of the island nation.

They were able to agree on a statement that the hostilities in Grenada have brought the action under the War Powers Amendment, which would require that the U.S. troops in Grenada be withdrawn unless Congress authorizes a longer stay.

Some congressional Republicans said the War Powers Amendment may not apply to Grenada.

O'Neill and other House Democratic leaders have declined to discuss Grenada while military action there continues. Democrats said O'Neill indicated in the party caucus that at some point he will blast Reagan for sending in soldiers and Marines.

Republicans in Congress praised the Grenada venture and predicted it will be a historic triumph for the United States. On the stationing of Marines in Lebanon, Republicans seemed to be backing Reagan strongly, although several said the sudden loss of American lives on Sunday there makes them edgy about future developments.

The House Democratic caucus yesterday was called to deal with demands from many Democratic representatives that the party reexamine its stance on Lebanon in light of the terrorist bombing that killed more than 200 American Marines.

Those dissident Democrats have two problems: First, their leader, O'Neill, is forcefully opposed to any further action to limit Reagan's prerogatives on the Marine force; second, they can't find a legislative vehicle that would succeed in getting the troops out of Beirut.

The House leadership has put off, probably until next Tuesday, any vote on a resolution that would cut off funding for the Lebanon deployment--which in effect would force Reagan to withdraw the Marines. That resolution is sponsored by Reps. Stratton and Long, a hawk and dove, respectively, on military issues who said they have come to agree that the Lebanon mission is a mistake.