As the rain fell misty and cold in this Atlanta suburb north of the Chattahoochee River, Kathryn Jackson McDonald sat for hours in her husband's small office with curtains drawn, behind his mahogany desk and official nameplate, across from the portrait of Gen. George Patton in battle dress.

Somber men came and went, whispering to her about plans for memorial services and offering fresh theories that the intelligence network of the John Birch Society had developed on how and why the Soviets killed her husband and their leader, Lawrence Patton McDonald, the anti-communist warrior and congressman from Georgia's 7th District.

Some withdraw to grieve in private after the loss of a loved one, but this young political widow did the opposite. In the three days since she learned that her husband was aboard the Korean airliner reportedly shot down by the Soviets, she presented herself to the press and public morning and night, saying that Larry McDonald had been targeted for assassination by the communists and that the world is at war. And with every passing hour she hinted more strongly that she would attempt to join the battle herself by campaigning to succeed her husband in the House in a special election this fall.

"I feel an obligation to carry Larry's banner," she said. "But in all honesty, I don't know how good I would be."

Larry McDonald, 48, was an extraordinary political figure in the stretch of Georgia running north from Atlanta to the Tennessee border, which he represented in Congress since 1975.

To say that he was a "southern Democrat" would not do justice to the fervor of his conservative ideology. As a leading spokesman for the anti-communist, conspiracy-oriented John Birch Society, of which he was elected national chairman earlier this year, he fought not just the communists, homosexuals, abortion and busing, but even forms of federalism, such as crop subsidies for farmers and the creation of the Chattahoochee River Recreation Area, that were of financial benefit to many of his constituents.

While his right-wing views and rejection of pragmatic politics made him an outsider in the give-and-take world of Washington, they served him well back home in Cobb County, a fertile, conservative territory that often has been compared politically to Orange County in California. The Atlanta newspapers attacked his policies year after year, and many Georgia Democrats complained that he had no loyalty to the party, but his outspokenness never lost its appeal to the voters.

"Eastern Cobb County is about as conservative as you can get," said Bert Lance, the Calhoun banker and Carter administration official who is now chairman of the state Democratic Party. "I had the sense that Larry was invincible there, that the seat was his for life."

Whether that appeal can be transferred to his widow is less certain.

The first question many Democratic leaders in the district are asking about Kathryn McDonald, 34, is whether she is even a nominal member of their party. Until meeting Larry when he gave a speech in her native Glendale, Calif., in 1974, she was a Republican Party activist and an official of the American Conservative Union.

"Even though the special election will be nonpartisan, we will have to know which party caucus she would join if she got to the House," said one Democratic official. "If it's the Republican caucus, you'll find a whole lot of Democrats running against her. There are at least five state legislators I know of who have wanted to run, but thought they couldn't beat Larry."

While publicly offering condolences to the McDonald family, many of those politicians were huddling privately with their advisers and placing dozens of telephone calls around the district this weekend to get an early line on the propriety and potential of challenging a young widow with two toddlers. Democrat George Pullen, a teacher at Floyd Junior College, has said he would probably run against her.

That a woman in the McDonald clan would enter politics in her own right shows that change comes to even the most traditional and patriarchal of families. When Larry McDonald and his older brother Harold were growing up, such a thing would have been unthinkable.

"In our family everything was autocratic," said Harold McDonald, who like his brother and their late father practiced medicine at the McDonald urology clinic here. "The father was the head, the mother was submissive and the children came to the table when they were called and didn't leave until they were told to. When our parents spoke to us, we said 'Yes, sir' and 'Yes, ma'am.' When we went anywhere, Dad always drove the car. And when we went out to eat, Dad usually ordered the food for everybody at the table."

According to his brother, Larry McDonald was shaped in his father's image. He never smoked or drank, and the Protestant work ethic was so deeply ingrained in him that he chastised his brother for attending weekend square dances when they were students together at Davidson College.

It wasn't until McDonald received his medical degree and went to Iceland as a naval surgeon, his brother said, that his puritanical instincts developed into a political ideology.

"Larry thought Iceland was run by communists and was deeply troubled that the U.S. government was accommodating them," said his brother.

In the 1960s, McDonald organized the John Birch Society chapter in Cobb County into one of the largest in the country, holding meetings in his living room once and twice a week, late into the night. By 1974, when he made his second race against the incumbent Democratic congressman, John Davis, the John Birch Society was such a significant political force in the district that McDonald won.

He soon emerged as one of the most articulate spokesmen for his cause. He joined forces with Rev. Jerry Falwell's Moral Majority, created his own anti-terrorist intelligence operation, Western Goals, in the Virginia suburbs of Washington, and traversed the country and the world decrying the communist threat.

At times he got into legal scrapes because of his full-throttle approach to the John Birch Society program. The family of a Birmingham, Ala., man, who had been given the apricot-pit substance laetrile by McDonald as a cancer treatment, sued him several years ago and won $15,000 damages. Prosecutors once accused him of stockpiling munitions by buying guns in the names of his patients.

But such problems seemed of minor concern to many of his bedrock conservative constituents.

"It's true that Larry wasn't a pragmatist, that he was zealous, and that he frustrated us now and then," said one Cobb County officeholder. "But, damn, with Larry you always knew where he stood."

This weekend, county politicians and ministers were beginning preparations for a memorial service for McDonald here in Marietta, the city that only weeks ago was rocking in celebration after winning the Little League World Series. Several ministers called Kathryn McDonald to offer their churches for the services, but she said she was thinking that the best place to hold them would be at the local civic center.

And in Washington, she and Falwell plan an even larger memorial service at Constitution Hall on Sept. 11. President Reagan has been invited. That same afternoon, a documentary funded by McDonald and the John Birch Society will be televised on the Turner Broadcasting System. The subject: communist subversion.