President Reagan, warning that Nicaragua has become a "command post for international terror" seeking to topple neighboring governments and give the Soviet Union a "beachhead in North America," urged Congress last night to approve $100 million in aid to Nicaraguan rebels for "the defense of our own southern frontier."

In a nationally televised address from the Oval Office, Reagan appealed for bipartisan support to send arms and supplies to the "freedom fighters" and declared that failure to do so could imperil Mexico and the Panama Canal, and send "desperate Latin peoples by the millions" fleeing toward the United States.

Describing Nicaragua's Sandinista government as an "outlaw regime," Reagan claimed that the country had become the "privileged sanctuary" for a host of America's enemies, including Cuba's Fidel Castro, Libya's Muammar Qaddafi, Iran's Ayatollah Ruhollah Khomeni, the Palestine Liberation Organization's Yasser Arafat, Italy's Red Brigades, and the war machines of the Soviet Union and its Warsaw Pact allies.

Opening the final days of his campaign for aid to the rebels, also known as contras or counterrevolutionaries, Reagan said Congress must decide: "Will we give the Nicaraguan democratic resistance the means to recapture their betrayed revolution, or will we turn our backs and ignore the malignancy in Managua until it spreads and becomes a mortal threat to the entire New World?"

Reagan did not discuss compromise proposals from Congress for aiding the rebels, and White House chief of staff Donald T. Regan predicted earlier yesterday that "we'll win" the president's original plan. Other White House officials said Reagan needs 15 to 18 additional votes to succeed in the House, where the first critical vote is scheduled Thursday.

Even as he described the stakes in Nicaragua in life-or-death terms, Reagan sought to allay the concerns expressed in polls by many Americans that he is leading the United States into a conflict similar to the Vietnam war. "Let me make one thing plain," he said. "I am not talking about American troops. They are not needed; they have not been requested. The democratic resistance fighting in Nicaragua is only asking America for supplies and support to save their own country from communism."

The president did not mention, however, that American troops across the border in Honduras would be used to train the rebels.

While Democrats have pressured the administration to delay military aid to the contras and attempt once more to find a diplomatic solution, Reagan said this approach has not worked.

"Ten times we have met and tried to reason with the Sandinistas," he said. "Ten times we were rebuffed. Last year, we endorsed church-mediated negotiations between the regime and the resistance. The Soviets and the Sandinistas responded with a rapid arms buildup of mortars, tanks, artillery and helicopter gunships." The president made no reference to the just-completed mission to Central America by his new special envoy, Philip Habib.

Reagan also did not refer to the public opposition of eight other Latin American nations to sending military aid to the contras. A senior administration official, briefing reporters at the White House, claimed that three of the "core four" neighbors of Nicaragua -- Costa Rica, El Salvador, Honduras and Guatemala -- have privately backed military aid to the rebels, despite their failure to do so publicly. However, the official refused to identify which nations had done so.

The president offered seemingly contradictory assessments of the military capabilities of the contras. With "their blood and courage," he said the rebels had "pinned down the Sandinista army and bought the people of Central America precious time." But he also said the "freedom fighters' supplies are running short, and they are virtually defenseless against the helicopter gunships" the Soviets have sent to Managua. Reagan said the contras' strength had "grown dramatically" since its inception in 1982, and today "numbers more than 20,000 volunteers and more come every day." He did not mention, however, that the administration's own estimates are that only about 6,000 rebels are actually fighting in Nicaragua; others say the number is less than half that many, and that the remaining contra forces have lost their military effectiveness.

The president compared the "freedom fighters" in Nicaragua to "the French Resistance that fought the Nazis" in World War II and described them as a "a factor the communists never counted on, a factor that now promises to give freedom a second chance."

Using charts, maps and photographs, Reagan devoted most of his address to describing Nicaragua's ties with the Soviets and others he said are seeking "an outcome deeply injurious" to the United States. "Will we permit the Soviet Union to put a second Cuba, a second Libya, right on the doorstep of the United States?" he asked. "It is not Nicaragua alone that threatens us, but those using Nicaragua as a privileged sanctuary for their struggle against the United States."

Referring to a map of Central America, Reagan said weapons supplied by the Sandinistas have been found in Honduras, El Salvador, Costa Rica and Guatemala, as the nations on the map turned red. "Radicals from Panama -- to the south -- have been trained in Nicaragua," he said, adding that the Sandinista "revolutionary reach" also extends to South America and the Caribbean. As more nations turned red on the map, Reagan said Nicaragua had provided military training, safe haven, communications, false documents, safe transit and "sometimes weapons" to "radicals" in Colombia, Ecuador, Brazil, Chile, Argentina, Uruguay and the Dominican Republic.

"Even that is not all, for there was an old communist slogan that the Sandinistas have made clear they honor: the road to victory . . . goes through Mexico," he said.

Reagan said that the Sandinista regime sponsors terrorism "that led last summer to the murder of four U.S. Marines in a cafe in San Salvador." However, the senior official told reporters the United States does not have evidence directly linking the murder to the Sandinistas. Reagan's statement, he said, was based on "the fact that the headquarters for the El Salvador guerrillas is in Nicaragua, and guidance and supplies come out of Nicaragua."

Reagan put strong emphasis last night on the value of Nicaragua as a base against the United States. He said the Sandinistas threaten Caribbean sea lanes that carry "almost half our foreign trade, more than half our imports of crude oil, and a significant portion of the military supplies we would have to send to the NATO alliance in the event of a crisis. These are the choke points where the sea lanes could be closed."

In Nicaragua, he said, "Warsaw Pact engineers" are building a deep-water port on the Caribbean coast "similar to the naval base in Cuba for Soviet-built submarines." Outside Managua, he said, they are building "the largest military airfield in Central America -- similar to those in Cuba, from which Russian Bear bombers patrol the U.S. East Coast from Maine to Florida." The Soviet bloc has given Nicaragua over $1 billion in aid, he said.

Reagan's proposal for $70 million in military aid and $30 million in nonlethal supplies to the rebels is designed to force the Soviets to "make a fundamental reassessment of their policy options in the Third World," according to the senior official who briefed reporters. If Congress defeats the plan, he added, "it will be more difficult to put U.S.-Soviet relations on a stable and constructive plane."

In his address, Reagan also harshly criticized internal repression, terrorism and drug trafficking by the Sandinistas. He displayed a photograph which he said showed a top aide to one of the nine Sandinista commandantes "loading an aircraft with illegal narcotics, bound for the United States." He described in graphic terms the torture of a pastor, and condemned the regime for revoking the civil liberties of Nicaraguans.

Reagan also said Managua's "only synagogue" was "desecrated and firebombed" by the Sandinistas and "the entire Jewish community forced to flee Nicaragua." This charge has been made in the past, though reports from Managua have quoted members of the tiny Jewish community there as saying the synagogue was closed voluntarily because it wasn't being used, and the Jews who left were among the businessmen close to the Somoza regime who departed Nicaragua when the Sandinistas took power.

Reagan closed the address with an appeal for bipartisan cooperation that mentioned the late Sen. Henry M. (Scoop) Jackson (D-Wash.) and Presidents John F. Kennedy and Harry S Truman. Just as Congress send aid to Greece under the "Truman Doctrine" to "save that country from the closing grip of a communist tyranny," Reagan said, "with that same bipartisan spirit we can save freedom in Nicaragua today."

"If we fail," he said, "there will be no evading responsibility, history will hold us accountable." White House officials said this line was carefully drawn to avoid giving the impression that Reagan was "finger-pointing" at Congress.

"I have only three years left to serve my country," Reagan said. "Could there be any greater tragedy than for us to sit back and permit this cancer to spread, leaving my successor to face far more agonizing decisions in the years ahead? . . . . We still have time to do what must be done so history will say of us, we had the vision, the courage and good sense to come together and act -- Republicans and Democrats -- when the price was not high and the risks were not great."

The Nicaraguan embassy yesterday issued a statement charging that the president is attempting to "hoodwink" Congress and the American people into suporting a policy of "creeping military involvement," of U.S. combat troops in Nicaragua. More aid to the contras, it said, is "incompatible" with efforts to seek peace through diplomacy.