Sen. Albert Gore Jr. (D-Tenn.) sharpened his attack on the foreign policy positions of his rivals for the 1988 Democratic presidential nomination yesterday, widening the first major schism over issues in the Democratic field as he stepped up his push for moderate and conservative support in the South.

"The politics of retreat, complacency and doubt may appeal to others, but it will not do for me, or for my country," Gore said in a speech to the National Press Club.

"I fear that our national party risks losing the faith and trust of the mainstream voters who have always supplied our mandate," he added, listing his support for the Reagan administration's policy of escorting ships in the Persian Gulf, the flight testing of ballistic missiles, and the funding of the Midgetman missile as being areas where "my fellow candidates and I see the essential needs of our national security in very different ways."

During the past 10 days, Gore has hammered away at those differences in two debates and two foreign policy speeches, attracting headlines and countercharges, separating himself from the field, and laying the foundation for a thrust he will make this fall to line up political support in the South.

"What you're seeing now is no accident," the candidate's father, former senator Albert Gore Sr. said in an interview. "We've got to get the Dixiecrats {southern conservative voters} back into the Democratic Party, and we're playing damn hard ball. Later in the campaign, after he has won the nomination, people will see the other side of his record come through: his work on the environment, on chemical waste, on standards for infant formula, on the ozone layer. It will be clear he is a very progressive, forward-looking candidate."

Gore is the only southerner in the Democratic field but, until now, his campaign has attracted scant political support outside his home state of Tennessee, this despite the stated preference of many southern leaders for a nominee from the South. "It was smart of him to do what he's doing now, because he was in danger of having his lack of political support in what's supposed to be his home region become a story," said Robert Beckel, former campaign manager for Walter F. Mondale, the 1984 Democratic nominee.

Gore is beginning to show movement in the South. Last week, he was endorsed by 20 Florida legislators, members of a group calling itself Committed '88. Yesterday, Florida Democratic Chairman Charles Whitehead said that Gore "spoke our language" at the Miami foreign policy debate Monday and predicted more support would go his way. Meantime, in the other southern megastate, Texas Democratic Chairman Robert Slagle said that "right now you can sense more movement for Gore here than for anybody else."

Fred Martin, Gore's campaign manager, said that a series of endorsement announcements throughout the South is planned for later this month. He emphasized that the foreign policy appeals were aimed not just at the South, but to show "an independence of thought" that would be attractive to voters all over.

Others disagreed. "Maybe the next debate should be between the old Al Gore and the new Al Gore," said Rep. Richard A. Gephardt (D-Mo.), another presidential candidate who accused Gore of changing his positions and creating an artificial distinction between himself and the rest of the field for tactical reasons.

Gore spokesmen reasserted yesterday that his House and Senate voting record is not at variance with positions he has staked out in the campaign. In the aggregate, however, the voting record also does not betray the sort of sharp ideological differences with other candidates that Gore has been touting. For example, Gore's lifetime vote rating from the Americans for Democratic Action, a liberal group, is 64.4 percent, compared with 60.5 percent for Gephardt and 75.5 percent for Sen. Paul Simon (D-Ill.), another presidential candidate.

Gore voted in 1983 for a nuclear freeze, as did Gephardt and Simon. But Gore also has supported the MX missile, chemical weapons, the B1 bomber and two additional aircraft carriers. He has cast some votes for -- and some votes against -- humanitarian aid to the contras in Nicaragua, and has never voted for military aid. He has opposed deployment of the Strategic Defense Initiative (SDI).

His record is essentially that of a centrist, and Gore has chosen in this campaign to give that record a hawk-like cast. His admirers said he has done so for reasons that go beyond mere positioning. "It's naturally where his mind is," said Stuart Eizenstat, former adviser to President Jimmy Carter, who has been giving advice to a number of Democrats.

Gore's detractors said he is enagaged in simple tactical politics. "He's going to hit a brick wall in a hurry," said Paul Maslin, Simon's pollster. "He's making a mistaken assumption about the southern electorate. He says he's shooting for the mainstream, but the mainstream has been saying we should be cutting the defense budget."