When Rep. Larry McDonald came to Washington from Georgia in 1975, one of the committees he said he would like to join was the House Internal Security Committee, which was the new name for the old House Committee on Un-American Activities.

But as luck would have it, House Democrats disbanded the committee in 1975, and McDonald, a longtime member of the John Birch Society, was left out in the cold, a Communist-hunter without a dragnet.

But not for long. A year and a half ago, McDonald created his own private foundation, Western Goals. Its stated mandate, among other things, is to "fight Communist-controlled penetrations and subversion" of the U.S. government and its institutions.

"As a tax-exempt organization," a recent foundation letter said, "[western Goals] will use every legitimate and legal means to halt and reverse our country's abdication of responsibility of the ideals and philosophy that contributed to our greatness of the past."

And the battle continues. Tucked away in a swank town house in Alexandria, McDonald's foundation, with a full-time staff of five, the help of an advisory group of retired generals and John Birch Society members, is conducting its own tax-exempt war on terrorism and what McDonald views as a worldwide Communist threat.

Its aim, Western Goals says, is to serve as "the first and only public foundation to . . . fill the critical gap caused by the crippling of the FBI, the disabling of the House Committee on Un-American Activities and the destruction of crucial government files." Already in place are books, newsletters and seminars, including ones on subversion in the Caribbean, Africa and South America.

McDonald says that the foundation's activities do not amount to an effort to bring back the investigations of the Joseph McCarthy era. "I thought witch hunts went out with Nathaniel Hawthorne," he said.

The information on "terrorism and subersion" that Western Goals wil accumulate and feed into its Alexandria computer will come largely from "public records" and the files of retired government employes "who choose to make their information available to us," he said.

McDonald, by profession a urologist from Marietta, also intends to monitor Communist activity in North and South Carolina (because he thinks Communist have made inroads there) and review "the crippling of U.S. intelligence agencies," and what a recent newsletter called "international terrorist solidarity."

In addition, his foundation regularly reports to its members on organizations it considers to be questionable.Among those watched are the Helsinki-based World Peace Council and the Institute for Policy Studies, Morton Halperin's Center for National Security Studies, both in Washington.

Western Goals' monthly newsletters also feature profiles of the foundation's 18-member advisory group, which includes novelist Taylor Caldwell, Rep. John Ashbrook (R-Ohio), Mrs. Walter Brennan, widow of the actor, physicist Edward Teller, retired Joint Chiefs chairman Thomas Moorer, and retired generals George S. Patton III, John Singlaub and Lewis Walt.

According to McDonald, the foundation, which has been granted tax-exempt status by the Internal Revenue Service, is working toward an annual budget of $2.5 million to$3 million after spending more than $1 million this year. It has sent out at least two solicitation letters this summer to potential subscribers around the nation, seeking donations in $25 to $1,000 amunts to fund the foundation's activities.

Among the things contributors are paying for is a computer for a file on "those who would seek to bring revolutionary change to America," and in general to "rebuild and strengthen the political, economic and social structure of the United States and Western Civilization so as to make any merger with totalitarians impossible."

Western Goals literature says that the organization's "veteran analysts will work closely" with official agencies, but "security reasons prevent us from telling you the details of this truly remarkable leap forward."

Western Goals is not the only private Washington group that monitors the activities of political organizations. Group Research Inc., at 419 New Jersey Ave. SE on Capitol Hill, specializes in assembling information about right-wing groups in this country.

The staff of McDonald's foundation is interlocked with his congressional staff.

Linda Catoe Guell, the director of the foundation, for which McDonald says she is paid about $20,000, also has a part-time secretarial position on McDonald's congressional staff. Before that she worked for 12 years in the office of Sen.Strom Thurmond (R-S.C.) as a special assistant.

The editor of Western Goals' publications is John Rees, whose wife, Louise, was a staff member of the defunct House Internal Security Committee and now works in McDonald's office as a researcher. The British-born Rees is a former police informant and now the Washington bureau chief of the John Birch Society weekly "The Review of the News." In addition, he puts out a biweekly intelligence newsletter called Intelligence Digest that has been supplied to police departments around the country.

Spokesmen for the Institute for Policy Studies and the Center for National Security Studies, research groups that have been the subject of Western Goals' reports alleging subversive activities, scoff at the foundation's charges, and deride the scholarship of its publications, including ones titled "Red Tide Rising in the Carolinas," "Ally Betrayed . . . Nicaragua," and "Broken Seals," a report on attempts to destroy the foreign and domestic intelligence capabilities of the United States.

"Most things in 'Broken Seals' are ridiculous twists of meaning," says Jay Peterzell of the Center for National Security Studies.

"It's hooligan journalism . . . John Rees is a master of innuendo, false premise, absurd conclusions . . .," said Robert Borosge, the director of the Institute for Policy Studies.