DALLAS, FEB. 18 -- The fight for "Super Tuesday" got off to a nasty start tonight when Sen. Albert Gore Jr. (Tenn.) went after the victors of New Hampshire and Iowa, challenging Massachusetts Gov. Michael S. Dukakis' defense credentials and Rep. Richard A. Gephardt's (Mo.) consistency on domestic policies.

Gore, who has staked his campaign on the March 8 voting in Texas and 13 other southern states, but who trails Gephardt in the early polls here, claimed that the Missouri congressman had changed his positions on abortion and taxes. "I'm going to lay it on the line, Dick," he said. "The next president has to be someone who the people believe will stay with his convictions."

Gephardt responded by saying that Gore had undergone his own convenient election-year transformation, moving to the political right after pulling out of Iowa and deciding to concentrate on the South.

"Lately you've been sounding more like Al Haig than Al Gore," Gephardt said.

"That line sounds more like Richard Nixon than Dick Gephardt," Gore shot back.

The debate started with Dukakis facing tough questions on defense issues.

"I'm not squishy soft. I'm very tough," he insisted when moderator Roger Mudd questioned his willingness to stand up for American interests. Dukakis said one way he would show his strength was by "never, ever making concessions to terrorists. No exceptions."

The sharpest exchange on defense issues was between Dukakis and Gore, who said that the Massachusetts governor's statements tonight sounded "a little different than what he said in Iowa on the eve of the Iowa caucuses." According to Gore, Dukakis said in Iowa that he would be willing to accept a Soviet client state in Central America.

"I never, ever said that. Please get your facts straight," Dukakis said. When Gore pursued the question, Dukakis added, "If you're going to be president of the United States, please get your facts straight."

The issue that so far has carried Gephardt, a tough trade policy that has been defined as "economic nationalism," was discussed only briefly, with former Colorado senator Gary Hart taking the role of Gephardt's chief critic.

In a line that drew gasps and laughs because of its multiple interpretations, Hart said of Gephardt's policy: "The road to hell is paved with bad . . . good intentions."

The Texas debate, on the campus of Southern Methodist University, featured a streamlined field, with former Arizona governor Bruce Babbitt withdrawing from the race earlier in the day and Sen. Paul Simon (Ill.), whose campaign is threatened by financial problems, choosing to make his stand in Minnesota and South Dakota.

Throughout the debate, Jesse L. Jackson won strong applause from the audience in his attacks on corporate mergers and the transfer of plants to overseas facilities. He specifically attacked General Electric, a company he said paid no taxes in recent years despite huge profits, while at the same time building products in Taiwan, not the United States.

Gore had been expected to use the Texas forum to begin a major attack on his opponents in an attempt to revive his campaign after sitting out Iowa and participating only marginally in New Hampshire. The creation of a single day on which 20 states would hold primaries or caucuses was designed to nominate a Democrat who supports "a strong America," he argued. "Super Tuesday, if it means anything at all, is designed to send that message home."

The trade issue was raised by Carl Leubsdorf, Washington bureau chief of the Dallas Morning News, who asked whether Gephardt's trade policy amounted to pandering to special interest groups in a fashion similar to the campaign tactics critics say got Walter F. Mondale in trouble in 1984.

"It's time we decided who we are for," Gephardt shot back. "I'm proud to stand up for American workers."

Leubsdorf then accused Gephardt of failing to provide a strong commitment to the 1986 tax reform bill, a measure Gephardt had sponsored. Gephardt denied the charge.

Hart, who trailed the field in both Iowa and New Hampshire, seemed to get a stronger reaction from the audience here than at previous debates in the North. But if anyone was the crowd favorite, it seemed to be Jackson, who avoided the nasty give-and-take that dominated the Dukakis-Gephardt-Gore trio. The best received line of the night was his. "Let us not allow Super Tuesday to become superficial," he said.