Rep. Lawrence Patton McDonald, the arch-conservative Georgia Democrat who was aboard the South Korean airliner shot down by a Soviet missile, was a hero to the American right wing.

Its leaders yesterday hailed him as a martyr to his cause.

At an emotional news conference, representatives of the Conservative Caucus and other conservative groups praised McDonald, condemned the Soviets, urged strong retaliation and suggested that the attack on the Korean jetliner was intended to silence McDonald and other conservative lawmakers.

The 48-year-old Marietta legislator, a veteran anti-communist crusader who recently became chairman of the John Birch Society, was one of a half dozen conservative lawmakers bound for Seoul to attend a three-day conference commemorating the 30th anniversary of the U.S. mutual defense treaty with South Korea.

But he apparently was the only member of Congress aboard the ill-fated Korean Air Lines Flight 007 when it strayed into Soviet airspace and apparently was shot down by a Soviet fighter. The others, including Sens. Jesse Helms (R-N.C.), Steve Symms (R-Idaho), Orrin G. Hatch (R-Utah), Edward Zorinski (D-Neb.) and Rep. Carroll Hubbard Jr. (D-Ky.), were arriving on other KAL flights.

It was only by a twist of fate that McDonald was aboard the downed craft and at least two of the others were not. McDonald's Atlanta-to-New York flight Sunday was diverted temporarily to Baltimore because of bad weather, forcing him to miss his New York-to-Seoul flight by "three to four minutes," according to Tommy T. Toles, his press aide.

As McDonald waited two days in New York for what turned out to be the doomed flight, Hubbard canceled a reservation on the same flight to attend a Tennessee Valley Authority symposium, according to aides. Hubbard then booked a flight through Nashville to Los Angeles, where he joined Symms and Helms on a Los-Angeles-to-Seoul flight Tuesday.

Symms also considered taking the Tuesday New York-to-Seoul flight, according to a Symms aide, but decided the Los Angeles flight was more convenient.

Helms, who had been in Houston, planned from the start to go through Los Angeles, aides said. Hatch and Zorinski were in Tokyo when news of the disaster broke early yesterday and planned to fly to Seoul later in the day, according to aides. Sens. Malcolm Wallop (R-Wyo.) and Robert W. Kasten Jr. (R-Wis.) and perhaps other lawmakers had considered attending the conference but decided against it, according to aides.

In another twist of fate, the New York and Los Angeles flights refueled in Anchorage at about the same time early Wednesday, and Hubbard reported that passengers of the two planes mingled in the airport lobby. McDonald's flight took off a few minutes before the other plane.

"The plane that Sen. Symms and Sen. Helms and I were on was about 10 minutes behind that particular plane and could have been shot down," a shaken Hubbard told a Cable News Network interviewer after arriving in Seoul.

"It is definitely unacceptable for the Soviets to do this," Symms, a close friend of McDonald, said in Seoul. "It is time for the United States to make a concrete reevaluation of its relationship with the Soviet Union."

At yesterday's conservative news conference, several speakers suggested that the Soviets shot down the plane to silence MacDonald.

"There is a real question in my mind that the Soviets may have actually murdered 269 passengers and crew on the Korean Air Lines Flight 007 in order to kill Larry McDonald," said Jerry Falwell, head of Moral Majority.

"What they had in mind was knocking out some of the foremost adversaries of communism," contended Rep. Philip M. Crane (R-Ill.).

Crane and Howard Phillips, chairman of the Caucus, said they had been told that the names of Helms and Symms were also on the manifest of the downed plane. Aides to Helms and Symms said, however, that they did not believe the two senators had made reservations on that flight. McDonald's staff also suggested that McDonald was the Soviets' target. "We think it was deliberate . . . . He was simply a danger the Soviets could not afford to face," said press aide Toles.

"This is a typical action of the communists and the Soviet Union," said McDonald's wife, Kathy. "It was against this type of behavior that my husband spoke out against so loudly and for so long."

McDonald was one of the most conservative members of Congress, perhaps the most conservative. A loner among his fellow legislators, he had little influence in either party and had made his mark among conservative groups outside Capitol Hill.

A former Navy flight surgeon and the father of five children, McDonald was elected to Congress in 1974 from Georgia's 7th Congressional District, stretching from the Atlanta suburbs to the Tennessee line.

But he had little in common with the post-Watergate Democratic "Class of '74," shunning their procedural reforms in favor of a crusade against communism and for many of the New Right's social concerns, such as banning abortions and fighting gun control.

A member of the Armed Services Committee, McDonald dwelt on what he regarded as the threat of international communism, writing extensively on the subject in the Congressional Record and elsewhere, including books.

In 1981, the House passed a McDonald amendment barring use of Legal Aid funds to "promote, defend or protect" homosexuality, even as the "Gay Men's Chorus" was performing on the Capitol steps. But another McDonald proposal to ban trade with the Soviet Union was overwhelmingly rejected.

On the outside, McDonald founded a tax-exempt foundation, the Western Goals Foundation, aimed at curtailing subversion, terrorism and communism in the United States. He also traveled around the country in recent years to campaign against the Panama Canal treaties and for a hard-line position in Central America.

He was chosen as national chairman of the John Birch Society last March, succeeding its founder and only previous chairman, Robert Welch.

McDonald had said his goal was to increase the society's membership, now estimated variously at 60,000 or 80,000 to 500,000.

In a statement yesterday that lauded McDonald and condemned the Soviet Union, the Birch Society urged that the United States break diplomatic and trade ties, contending that Soviet technological advances, gained from U.S. planes captured in Iran after overthrow of the shah, "helped the Soviets to perpetrate this and other outrages."

As a physician and practicing urologist before his election to Congress, McDonald advocated research in use of Laetrile for treatment of cancer in the early 1970s. In 1978, a federal court jury awarded $15,000 to the widow of a cancer victim who was treated with Laetrile but found in favor of McDonald and an Atlanta hospital in other verdicts in a $6 million suit.

The conference for which McDonald was headed was jointly sponsored by the Washington-based Institute of American Relations, a private tax-exempt foundation set up by two aides to Helms to help fight international communism, and the Korean-based Asiatic Research Center of Korea, which congressional aides said was affiliated with the University of Korea.

Other scheduled speakers included the president of Korea and the U.S. ambassador to Korea, according to literature on the conference.

At the conservatives' news conference, Rep. George Hansen (R-Idaho), holding up a small replica of a Soviet fighter, said the tragedy was nonetheless a "red letter day" for conservatism in that it rallied disparate conservative groups behind the cause for which McDonald stood.

"The tragic death of Congressman Larry McDonald will have served a noble purpose if it exposes the mindless brutality of the Soviet Union for what it is," added Paul M. Weyrich, director of the New Right's Committee for the Survival of a Free Congress. John T. Dolan, chairman of the National Conservative Political Action Committee (NCPAC), went farther, suggesting that some of McDonald's congressional foes were also responsible for the tragedy.

"The apparent death of McDonald and the other innocent victims of Soviet aggression is directly attributable to colleagues of Congressman McDonald who believe in appeasing the Soviet empire. The Soviets have literally gotten away with murder within the last 15 years because no one would listen to Larry McDonald's warnings," Dolan said.

Added McDonald aide Toles: "We think of him as the first victim of World War III."