President Reagan scored the unusual political trick last night of disappointing the New Right but winning support from Democratic members of Congress in reaction to his speech on the shooting down of the South Korean airliner.

On an evening when much of political Washington was concentrating on the Labor Day holiday and the Redskins game at RFK Stadium, the most pungent response to the president's speech came from Richard A. Viguerie, publisher of Conservative Digest and a fund-raiser for New Right causes.

Vigerie said "it looks like" Reagan has "put Teddy Roosevelt in reverse: he speaks loudly but carries a small twig."

Reagan's speech was full of tough language, but suggested limited diplomatic sanctions against the Soviets in reaction to the incident, which claimed 269 lives.

"I find it hard to believe that I was watching Ronald Reagan, the 25-year leader of anti-communism," Viguerie said. " . . . I expected the president to be soft, with all the leaks that had come out, but I didn't expect him to be this soft." Viguerie called the president's proposals "namby-pamby" and said, "I don't think this is going to wash with the American people."

Paul Weyrich, director of the Committee for Survival of a Free Congress, a New Right group, was equally disenchanted. "I am very disappointed that President Reagan missed this great opportunity to exercise decisive leadership," Weyrich said. "I fear that the voters may judge him the way they did President Carter's handling of the Iranian crisis . . . . "

Weyrich and others have called on Reagan to close U.S. ports to Soviet shipping, cancel all arms control negotiations, expel Soviet diplomatic personnel and void the recent grain agreement.

Senate Minority Leader Robert C. Byrd (D-W.Va.), while generally supportive of Reagan, also mentioned the grain agreement as a possible pressure point on the Soviets.

"The president was tough in tone but restrained in his actions," Byrd said. "While I am fully supportive of the measures he outlined, I would have preferred the administration attempt to work in concert with other nations in imposing stiff trade sanctions. I would have hoped the president would have suspended the grain deal at least until the Soviets admitted shooting down the airliner and said they were willing to compensate the families of the victims."

Sen. Gary Hart (D-Colo.), one of those seeking the Democratic presidential nomination, also supported Reagan's actions, calling them "the right approach and response to the Soviet incident." However, Hart called Reagan "misguided in tying the incident to the production and deployment of the MX missile."

Former Florida governor Reubin Askew, also a Democratic presidential hopeful, said, "The president has offered a reasoned response to the unspeakable and totally inexcusable action of the Soviet Union. In particular, I agree with him that we must continue to seek arms control . . . because arms control is very much in our own national interest and in the interest of all the world."

Sen. Richard G. Lugar (R-Ind.) said, "President Reagan was strong in his description of Soviet barbarism and relentless in his forceful push for Soviet apology, payment to victims and international action which will embarrass and push the Soviets without inflicting equal harm on other countries."