HOUSTON, OCT. 28 -- Vice President Bush fought off attacks from his Republican rivals tonight in the opening debate of the GOP presidential campaign, standing alone as a defender of President Reagan's prospective treaty with the Soviet Union eliminating medium- and shorter-range missiles in Europe.

Direct attacks on Bush by former Delaware governor Pierre S. (Pete) du Pont IV were met with boos. And the hall was gripped with silence when, in a sharp exchange over the arms treaty, former secretary of state Alexander M. Haig Jr. lashed out at Bush with, "I never heard a wimp out of you" about the treaty when they were Reagan Cabinet colleagues in 1982.

Senate Minority Leader Robert J. Dole (R-Kan.), considered Bush's chief rival for their party's nomination, was also a target of criticism tonight but sought to play the role of peacemaker, throwing in quips whenever the debate got tense.

Rep. Jack Kemp (R-N.Y.) and former television evangelist Marion G. (Pat) Robertson, in aggressive presentations, stressed their conservative agendas while Dole and Bush emphasized their government experience and leadership roles.

The topics of arms control and the stock market collapse dominated the two-hour "Firing Line" confrontation carried by Public Broadcasting Systems across the nation. Host William F. Buckley Jr. was joined by former Democratic National Committee chairman Robert S. Strauss in questioning the six Republicans in their first joint debate of the campaign.

Bush, describing himself as "the copilot" of the Reagan administration, said it is easy for those outside the administration to indulge in "carping and criticizing, but it's different to be in there making the tough calls." That comment, like several others of the vice president, drew strong applause.

The debate came to life when Buckley asked about the prospective intermediate-range nuclear forces (INF) treaty and Bush found himself the only all-out defender of it.

Haig, Kemp, Robertson and du Pont expressed strong disagreement with the pact, saying it would increase the risk of conventional war in Europe and let the Soviets escape responsibility for violating past treaties and pursuing repressive policies at home, in Eastern Europe and in Afghanistan. Dole called for "a little dose of healthy skepticism" about its merits. "We shouldn't be out there cheerleading for the treaty until we know what's in it," he said.

Bush, answering last on this round, did not back off. He said every European leader he had met on his recent European trip supports the treaty and so do most Americans.

Addressing Kemp by name, Bush said, "Our president has stayed firm with the Soviets. And for the first time we are getting rid of an entire generation of nuclear weapons. And that's good for my grandchildren and for the whole world."

That was not the end of it, however. Although the next question invited comments on Robertson's impact on the Republican Party, du Pont turned back to the INF question and said Bush's answer "illustrates the concern many feel about where you would lead America. We've not seen any vision, any principle, any policy. We're waiting for details, and we're hearing generalities."

Du Pont was booed by some in the audience, and Bush said, "Pierre, let me help you." Then he ripped into du Pont's proposal that young people be offered incentives for establishing private retirement accounts as an alternative to Social Security, saying, "It's a nutty idea to fool around with Social Security. It's a new idea, but it's a dumb idea."

When Bush reiterated the Europeans' support for the INF treaty, Haig said, "I've just come back from Europe, too, and they are unsettled by this treaty." He said the professed support resulted from "arm-twisting" by the Reagan administration.

Bush broke in to say, "Al, you supported this in 1982."

Haig, explaining he had argued against the proposal when it was being discussed in Cabinet meetings, turned to Bush and said, ". . . I never heard a wimp out of you, George."

"I came here tonight to be nice and polite," Dole said in one of a series of quips on camera and off. "This debate is starting to liven up. I was afraid it was going to die on the vine."

Decorum was restored in the next round when all the candidates except Haig expressed strong support for the Strategic Defense Initiative and vowed to push it ahead. Haig questioned the others' criticism of reliance on nuclear deterrence, decrying "pie in the sky" alternatives. Questioned by Strauss about the stock market's dive and the budget negotiations under way in Washington, Bush insisted twice that he would not raise taxes, despite Reagan's recent comments that he would consider a tax increase as part of a deficit-cutting package.

"As president I will not raise taxes," Bush vowed. His promise was echoed by Robertson and Kemp, but Dole emphasized his role in fashioning a compromise today.

Dole, who has long differed with Kemp on the deficit issue, criticized Kemp directly in responding on the issue. "We're not going to grow out of it as Jack used to say," Dole said, referring to Kemp and others, including Reagan, who have said economic growth would shrink the deficit.

Dole called for Japan and West Germany to stimulate their economies "dollar for dollar" to match the fiscal restraint imposed in this country. Dole, who has indirectly criticized Reagan's lack of action in the wake of the market's plunge, also called for a "world economic summit" and said, "We've got to get this done."

This drew a barb from du Pont, who said that putting off action on the deficit until after Japan acts is "nonsense."

Robertson said he would not raise taxes, tariffs or interest rates and insisted that a president has to stop "playing games with special-interest groups."

The candidates also expressed similar views on abortion, saying that they oppose it and pledging support for a constitutional amendment to outlaw it. Robertson went further, saying that he would seek to deny federal funds to Planned Parenthood. But Bush said he favors sending family-planning information to other nations, saying it has "nothing to do with abortion."

The Republicans who seek to succeed Reagan were asked by Strauss whether they subscribe to Reagan's antigovernment philosophy. They appeared to be divided on the subject.

Dole and Bush argued that they would not attack government as Reagan did. "Government is here to stay," Dole said, urging more attention to "tightening up" programs and adding that while Americans wanted a Washington outsider in the elections of 1976, 1980 and 1984, "in 1988, Americans are looking for an insider."

"I'm not antigovernment," said Bush, who recalled that he had met the "finest public servants" when he was director of the Central Intelligence Agency and in his other government positions. "I'm not one who tears down those who serve" the country, he said.

Du Pont drew one of the biggest rounds of applause from the audience of Republican partisans when he used the question to attack those who opposed the unsuccessful nomination of Robert H. Bork to the Supreme Court. "Those of us in the conservative majority watched Robert Bork be savaged . . . and that's not in keeping with our values," he said.

Haig, who earlier in the debate criticized Reagan and his advisers, used the question to demand what he described as "reorganization" of Congress. Currently, he said, Congress is "one-man, one-vote anarchy," adding that unlike the consensus politics of earlier days, "today the most outrageous radical" gets on television and gets reelected.

Asked to criticize one of their competitors, the candidates turned to humor and substantive attacks. Kemp, for example, admonished du Pont for statements that the solvency of the Social Security system is in doubt. Kemp said the contention is an "absolute falsity" and the system "is not broke" but is running a surplus. Referring then to du Pont's remarks on eliminating farm subsidies, Kemp portrayed his conservative rival as a candidate experimenting with ideas. "We don't know which libertarian or market-oriented solution you're going to come up with next," Kemp said.

At the outset of the forum, four of the six contenders signaled their loyalty to Reagan by saying that they would hang his picture prominently in the White House if they were his successor. Haig and Robertson omitted Reagan from their personal Pantheon, although Robertson later specified that he had "great admiration" for the president.

Haig compounded the heresy a moment later when he took the invitation from Strauss to specify what he would have done differently from Reagan in the past seven years.

The question went first to Bush, who drew strong applause by saying, "I could" spell out his differences with Reagan "but I won't . . . . In our family, loyalty is a strength, not a character flaw."

Haig, answering next, reprimanded Bush by saying that loyalty means "telling the man you work for what he needs to hear, not just what he wants to hear. . . . There have been mistakes made." He cited the budget deficit and dealings with the Soviet Union.

Kemp said Reagan had made a mistake in accepting the 1982 tax increase pushed through the Senate by Dole and in delaying a commitment to early deployment of SDI.

Dole portrayed himself as a Reagan loyalist in the Senate, but said the GOP has a problem in a perception of "insensitivity" to the concerns of minorities and poor. Robertson cited "the enormous deficit and the loss of our competitive edge" as challenges for the next president, and du Pont added the funding of Social Security benefits for the Baby Boom generation to the list of unmet problems.

In their closing comments, Bush, Dole and Haig stressed their claims to strong leadership, while Robertson, Kemp and du Pont emphasized the particular issues and values they have put at the center of their campaigns.

"I have made a difference. I can make things happen," Dole said. "Tough times need a tough leader," Haig said. "I've been copilot for seven years," Bush said, "and I know how to land the plane in a storm."

Robertson, stressing family values, said it is time for America to "get back to basics." Kemp promised full employment and strong defense, emphasizing that he alone supported early deployment of SDI. And du Pont ticked off his proposals for welfare changes, drug testing, education vouchers and a phase out of farm subsidies.