The disaffection of a steadfast source of support for Jimmy Carter was signaled in the heart of what supposedly is still Carter country by Dr. William W. Pennell, pastor of the huge Forrest Hills Baptist Church in Decatur, Ga.

"I've had about all the born-again diplomacy I can stand," he declared in his rich baritone. That reference to the conduct of foreign policy by President Carter, a born-again Christian, was greeted with laughter, applause and a few shouts of "Men, brother" by his audience: some 200 Georgia preachers.

What makes this so disquieting for the White House is that Pennell admittedly was a staunch supporter of Carter in 1976 (ironically, while living in Gerald Ford's Michigan). Whether they admit it or not, so were most of the clergymen listening to him.

The nature of their unpublicized, invitation-only meeting in Atlanta also ought to disturb Carter. These Protestant preachers, mostly evangelical and fundamentalist but from many denominations, were gathered to get organized for political action from the pulpit unprecedented in this nation.

Their purpose is plain: to influence the vote of their parishioners from the courthouse to the White House. While no formal presidential endorsement is being made, there is no doubt that they will be telling their flocks to vote for Ronald Reagan. Only Reagan's selection of a clearly unacceptable running mate (Sen. Howard Baker or possibly, but not necessarily, George Bush) might avert this.

Although the politicization of the preachers is publicized at the level of nationally televised evangelists, its greater impact may be at the grass roots. Two more sessions such as the Atlanta meeting will be held in Georgia, and those meetings in turn will be duplicated across the nation.

The day-long session in Atlanta started with exhortations on issues (conservative social policy, pro-defense buildup) and ended with nuts-and-bolts advice on how the ministers can tell their flocks how to vote without losing their tax exemptions. Through it all, not much was said about Carter, but what was said was all negative.

"New right" political activist Paul Weyrich, the lead-off speaker, predicted the president, would veto any school-prayer bill passed by Congress. Dr. George Stanley, pastor of Atlanta's First Baptist Church, broke in to say that Carter had left no doubt whatever of that intention at a meeting with evangelicals earlier this year.

Do any of the clergymen attending the political action session here still back Carter? "None," one small-town Georgia Methodist minister informed us. "The problem is to get any of them to confess they voted for Carter last time. But believe me, they did."

Pennell is not reluctant to confess those past ties. "I was thrilled at the thought of a born-again in the White House," he told us. But the thrill began to fade on inauguration day when Carter "mentioned the Lord's name less often than any other president." Pennell flatly predicts that church opposition will cost Carter even his native Georgia, a state politicians regard as overwhelmingly the president's.

The preachers feel Carter has betrayed them on the social questions they care about most deeply, as stressed in the Atlanta conference: abortion, the Equal Rights Amendment, homosexuals (usually referred to as "queers" during the meeting here) and school prayer. There was no talk about economics or taxes. While "socialism" was never mentioned, "humanism" was anathematized repeatedly.

So was Carter's defense policy. The need for greater military spending was a keynote at a meeting where several ministers mentioned "just 1,000 days remaining" for the American republic to save itself from communist domination. While not related to apocalyptic Biblical prophesy, this warning reflects the conviction by many evangelicals that this world is in its "last days."

Can these preachers, new to politics and naive about it, sway their congregations? Here in the Bible Belt, chances are good.

Even before the Atlanta meeting, Georgia clergymen were passing out to parishioners a tract called "Ronald Reagan: A Man of Faith." In it, Reagan supports private Christian schools and school prayer while attacking abortion and homosexuality. "The time has come to turn to God and reassert our trust in Him for the healing of America," he says. That spells big problems for Carter among voters he cannot afford to lose.