President Reagan yesterday put new stress on what had previously been mostly a backup argument for his policies in Central America, saying that victories by leftist revolutionaries there could produce what he called "a tidal wave of refugees" flowing northward and threatening the United States with economic, social and even military disruption.

In discussions with congressmen, in recent speeches like Reagan's last night and in various written analyses, administration officials have been echoing this refugee-threat theme.

"One of the most significant differences between Central America and Vietnam," United Nations Ambassador Jeane J. Kirkpatrick told an audience of members of Congress 10 days ago, "is the Pacific Ocean." The United States withdrew from Vietnam "without feeling any direct national consequences," but it would be different if we withdrew from Central America, she said. "The refugee problem . . . is smaller now than it could be later."

Skeptics on Capitol Hill have noted that few refugees fled Nicaragua after its 1979 takeover by leftist Sandinista rebels, and that the 1 million refugees from Fidel Castro's Cuba were mostly middle-class entrepreneurs who generally have been no burden on the U.S. economy.

"The historical record just doesn't support this thesis," an aide to one Republican senator said of the administration's refugee rationale. "I hope they don't lean too hard on this one."

"What we would do about it is never spelled out," said another Senate staff member. "They just raise this image of a brown horde and we're supposed to shudder and go along with what they want."

Rep. Dante B. Fascell (D-Fla.), chairman of the Foreign Affairs subcommittee on international operations, said, "Refugees have been a problem for 30 years, but they don't have a thing in the world to do with Cuba or communism or any of that. It's economic."

However, at the office of the White House coordinator for refugee affairs, assistant coordinator Richard Krieger, author of a congressional briefing paper on the subject, said, "We're trying to alert the Congress, to alert the people what's going to happen to the country."

Basing his estimates on the number of people who fled Cuba after Castro's takeover, he said that 8 to 10 percent of the population of each country in Central America, a total of up to 2.3 million refugees, might "take flight to escape a Marxist takeover." This would produce "tremendous havoc and dislocation, social imbalance that would create political upheaval in this country," he said.

Illegal immigrants now move almost at will back and forth across the 2,000-mile Mexican border, frustrating most efforts by U.S. immigration law enforcement officials to stop them. Pending congressional legislation to cope with America's resident illegal aliens makes almost no attempt to plug the holes in the fence, opting instead to make it harder for undocumented aliens to get work. Officials said this would not deter refugees fleeing communist advances.

One Army general has been warning his colleagues that Central America could deteriorate to the point that U.S. troops would have to be withdrawn from NATO to halt the refugees at the Mexican border. Gen. E.C. Meyer, Army chief of staff, said "there is a group" within the U.S. military community "who feels that this is a possible follow-on to the turmoil that is taking place in Central America."

There is no evidence that the administration ever has contemplated fortifying the southern border, however. House and Senate intelligence committee members said that, to their knowledge, there has never even been a study of what it would require and it probably would be futile to try.

"If they really started coming, you could never stop them there. It has to be done at the source, in Central America," said a state department official specializing in that region.

"We raise it every time we can when it's appropriate," he added. "When people are saying the roots of the problem are social and economic, we say, 'Yeah, don't forget the external support, and don't forget our own interests--whether we can absorb all the refugees.' "