Sen. Jim Sasser (D-Tenn.), in the official Democratic response to President Reagan's appeal for aid to Nicaraguan rebels, said last night that the president is "seizing military options before he has exhausted the hope of a peaceful solution" and warned that U.S. troops will ultimately be required if Reagan pursues a policy of armed conflict.

Most members of Congress and most Americans agree with the president that Nicaragua must never become "a base for Soviet military adventurism," he said, but they want to try different means of preventing it.

"We believe the U.S. should grasp the initiative, seeking peace through negotiations before taking a fateful step which could lead to war in Central America . . . . This is the heart of our difference with the president. He proposes a wider war in Nicaragua, now," Sasser said.

And he added, "As the father of a 17-year-old son, I say, Mr. President, let's not rush into that quagmire. We've done that before."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said that Sasser spoke "for the average American parent. He cut to the heart of the issue -- whether the United States should become militarily involved in the war in Nicaragua."

O'Neill added that Reagan's remarks "sounded like a declaration of war against one of the smallest, poorest nations in the hemisphere."

According to Democratic aides, Sasser's response sought to express "mainstream" U.S. thinking, rather than more extreme or ideological party positions offered in years past.

Sasser "was the best respondent we've had since Edmund Muskie in 1970," said an aide to O'Neill, referring to a Democratic response by the former Maine senator to President Nixon on the issue of the Vietnam war.

In his response, Sasser said: "If the president's goal in Central America is the military overthrow of the Sandinistas, he should tell us so, because that goal simply cannot be achieved without direct U.S. military involvement in a long and costly and bloody war."

Sasser is author of a proposal, rejected by Reagan, that Congress approve the military aid but withhold it for six months while efforts are made to arrange peace talks between the Sandinistas and the rebels. The White House attitude toward this and other suggestions hardened after Republican congressional leaders complained that the talk of various "compromises" had undermined efforts to win passage of Reagan's package.

As of last week, O'Neill said, the proposal would have lost by about 25 votes. Prospects in the Republican-controlled Senate were described by Sen. David F. Durenberger (R-Minn.) as "at best 50-50." The House vote is scheduled for Thursday.

Speaking of the Nicaraguan insurgents, Sasser said, "The president calls the contras 'freedom fighters' and has even compared them to our own founding fathers. In fact, most contra military leaders fought against freedom as members of the Somoza regime's hated security force."

Sandinista atrocities spoken of by the president "certainly exist," he said. But "by all unbiased accounts, they pale beside those of the contras . . . . Neither side has clean hands in this war."

Most members of Congress "agree that the Sandinista government has betrayed the promise of its own revolution, has suppressed the freedom of its own people and has supported subversion . . . in El Salvador," he said.

"We agree that Nicaragua must never become a base for Soviet military adventurism in this hemisphere. Never."

Charging that it was Reagan who "broke off negotiations" with the Sandinistas last year, and noting that they "now claim a willingness to resume these talks," he said, "let's put the Sandinistas to the test."

Democrats have drummed on the theme that the administration's policy toward Nicaragua risks the involvement of U.S. troops, citing the officially reported belief of the intelligence community that only U.S. forces can resolve what otherwise will be at best a bloody stalemate.

In other Democratic reaction last night, Sen. Claiborne Pell (R.I.), ranking minority member on the Foreign Relations Committee, called Reagan's speech powerful "but divisive in effect and almost scary in tone. In fact, not a single neighbor of Nicaragua's except for El Salvador has publicly supported" the president's proposal.

Sen. Alan Cranston (D-Calif.) said Reagan's approach recalls "the precise pattern we followed in Vietnam -- money followed by military advisers followed by American troops."

Rep. David E. Bonior (D-Mich.), a leading opponent of the military aid request, said Reagan is not looking for diplomatic support from other nations in Central America. "The tone was clearly one in which the president seems ready to commit troops in Nicaragua," he said.