President Carter crisscrossed his native South today, pleading for Election Day help from his fellow-southerners and lashing out at what he called "vicious television ads that question my religious beliefs."

From the magnolia-lined grounds of the governor's mansion in Columbia, S.C., to the rich Mississippi Delta country near here, the president spoke of his pride in his southern heritage and warned his supporters the outcome of the election is now "very much in doubt."

The euphoria that existed in the Carter campaign organization immediately after Tuesday's debate was largely absent today.

In Memphis just before arriving here for a rally, Carter brought a "town meeting" audience to its feet cheering with a defense of his religious beliefs and his adherence to traditional Christian values.

"Until this year, I never had anyone question my belief in God," he said, adding:

"Here in the last few days of an election to have my opponent and those who support my opponent allege that I have a false belief or that I would twist my beliefs . . . is very, very disturbing."

Carter was referring to a series of television commercials being broadcast in the South by fundamentalist Chritian supporters of Ronald Reagan that attempt to link him with homosexuals and abortion advocates.

The president told the hushed audience in a hangar at the Memphis airport he believed that there should be "no religious test for political acceptability," and "no political test for religious fellowship."

Reagan is receiving strong support from fundamentalist Christian organizations such as the moral majority. Carter has tried to counter this by denouncing "religious intolerence" throughout the campaign, at one point suffering a backlash himself when he suggested that Reagan's election might result in the separation of "Jew from Christian."

But his comments today were his strongest and most direct on the subject and his grim demeanor while discussing the fundamentalist assualts on his moral convictions suggested these attacks are hurting him, personally if not politically.

As Carter campaigned across the South, there was a new sense of urgency in his appeals to the people of his natural geographic and political base. His aides conceded there has been "a pause in the momentum" for the president, and for the first time said they now believe it was Reagan who profited the most from the presidential debate in Cleveland earlier this week.

The Republican nominee appears to have received "a little help out of the debate, at least in the short term," a senior Carter aide told reporters. "He came across as appearing much more credible than his statements actually were -- as we're all now discovering."

In addition, this official said the president appears to have been damaged by this week's rise and then abrupt decline in optimism over a possible release of American hostages in Iran.

Carter campaign officials have said throughout the week that the race is so close in so many key states it is impossible to predict the outcome.

Generally large and enthusiastic crowds greeted the president along his southern tour today. But Carter, while looking confident, began to sound today like a man with growing doubts as the election nears.

"Without your help, I cannot win," he told a rally in Lakeland, Fla., this morning. "If the election were held today the issue would be very much in doubt. It's a close election."

There may have been a degree of political calculation in these assessments that Carter's momentum has slowed. More than anything, the president now needs a big turnout in traditional Democratic strongholds, which his warnings about the closeness of the race were clearly designed to provoke.

Carter's day began in Columbia, S.C., where his policies toward the Soviet Union were subjected to some hostile questioning at an early morning "town meeting."

"I'd like to point out that in recent years we have not fared poorly in our contest with the Soviet Union," he said. Carter said the Soviet invasion of Afghanistan "was an indication of the failure of communism," and argued that while the Soviets have alienated many countries around their borders the United States has gained new friends in China and Africa.

From Columbia, the president flew to Florida where it became clear his main message on this Halloween day was to be about the hobgoblins in Reagan's past.

High over Florida, Reagan's voice boomed through the Carter press plane with a warning that Medicare was only a first step toward "socialized medicine." The voice came from a 1961 record made by Reagan for the American Medical Association that was played for reporters by delighted White House aides.

To buttress this message, the president's campaign entourage included former heath, education and welfare secretary Wilbur Cohen and former commissioner of Social Security Robert Ball, who said Reagan was wrong about Medicare in the 1963s and did not appear to understand the workings of the Social Security system in the 1980s.

In Lakeland, Carter charged Reagan had "misrepresented" his past positions on Social Security during their debate and may have undermined public confidence in the system by erroneously saying young people would never get out of Social Security as much as they put in.