President Carter took his self-proclaimed less strident campaign style to the South today but his central message about the dangers of a Ronald Reagan presidency remained the same.

At an airport rally near Bristol, Tenn., this morning, and later at a "town meeting" at the Grand Ole Opry here, Carter hit hard on previous Reagan statements suggesting U.S. military involvement abroad.

"Time after time my opponent in the last few years has called for the injection of American military forces into diplomatically troubled regions of the world -- in Korea, Ecuador, Cuba, the Middle East, Pakistan, Angola," he told thousands of people who greeted him at the Tri-Cities airport in East Tennessee.

"Time after time when there was a problem, my opponent has said, 'Let's send American military forces there to settle the dispute.' Fortunately, Democratic and Republican presidents have not done that."

The voice was lowered but the campaign themes remained unchanged, as Carter criticized Reagan's stand on nuclear arms control and chided the Republican nominee for suggesting that the U.S. military has grown weak.

"In every campaign when a candidate charged that the Russians were ahead of us, after the campaign those charges were proven to be false," Carter said. o"A perfect example is my Republican opponent's recent suggestion that the United States' weakness is what caused us to stay neutral in the war between Iran and Iraq. It's a sign of weakness if you have to get involved military in a combat."

For a candidate trying, as Carter described himself in a television interview Wednesday night, to get his campaign "back on track," the day began well. The weather was bright and sunny and the crowds, both in East Tennessee and at the Grand Ole Opry, were large, friendly and responsive as the president reminded them that he shares their heritage as southerners.

Carter's more controlled rhetoric was the result of a mid-campaign reassessment by his political advisers. With less than four weeks until the election, surveys show the president still trailing Reagan, especially in projected number of electoral votes. Moveover, it had become increasingly clear in recent days that Carter's often strident attacks on his opponent were backfiring and failing to inject his own campaign with any sense of momentum.

One result of the reassessment became immediately clear today: campaign themes that had been stated negatively earlier are now being put in more positive terms. For example, on Monday in Chicago the president said Reagan's election might mean the separation of "black from white, Jew from Christian, North from South, rural from urban."

Today he dropped any reference to religion and said, "I want to see the nation united, North and South united, black and white united, rural and urban united."

From Tennessee, Carter flew to Winston-Salem, N.C., and ended up tonight in Tallahassee, Fla. This was his third campaign trip to his native South, his political base, where Reagan's conservative views are popular.

Carter carried Tennessee easily in 1976, but this year, according to Jim Hall, his state campaign coordinator, his standing in the state is "shaky"; still, Hall said he expected Carter to take the state on Nov. 4.

There was no evidence of political trouble in the receptions Carter received today. The most critical question at the "town meeting" dealt with campaign tactics. The president was asked by a high school student why he had "lowered yourself to the extent of slinging mud and making slanderous statements with your opponent."

Carter conceded that the campaign "has gotten sidetracked," but he said, "I have never intended to criticize Gov. Reagan personally."

Carter also criticized the press, particularly the television networks, for ignoring the issues and contributing to the lowered tone of the campaign.

"They covered the technique of the capaign and who is going to debate whom and all that kind of stuff," Carter said.