HANOVER, N.H., JAN. 16 -- In the most freewheeling debate yet of the intensifying presidential campaign, six Republicans heaped criticism on each other today in a two-hour televised encounter that featured a newly aggressive Sen. Robert J. Dole (R-Kan.) and frequent assaults on the front-runner, Vice President Bush.

In their fourth televised confrontation, a month before the New Hampshire primary, the Republicans quarreled over ethical lapses, the Iran-contra affair, Social Security, drug abuse, U.S.-Soviet relations, nuclear power, the Middle East, Central America, agriculture and, most of all, their records and potential as president.

While none of the six aspirants was spared scrutiny in the Dartmouth College debate, skillfully moderated by John Chancellor of NBC News, it was Dole who showed a different approach from his earlier appearances. He needled Bush repeatedly over such issues as student-loan funding, what Dole described as the vice president's absence from Social Security negotiations in 1983 and recent Bush campaign tactics in circulating a newspaper clipping that raised questions about Dole family finances. Dole also hit Bush's call for the release of 10 years of tax returns by announcing he had released 21 years of returns today.

Sen. Warren B. Rudman (R-N.H.), who heads the Dole campaign here, said afterward that the senator "finally overcame '76" in his aggressive stance, a reference to Dole's sometimes slashing attacks as Republican vice-presidential nominee that year. "He was himself tonight," Rudman said. But others noted that Dole showed flashes of stern anger as well as wit.

The others livened up the debate with spirited rhetorical battles.

Former Delaware governor Pierre S. (Pete) du Pont IV drew scorn for his claim that the Social Security system faces long-term financial troubles and for his plan to deprive teen-agers of driver's licenses if they use illegal drugs.

Former television evangelist Pat Robertson got into a heated exchange with Bush over terms of the recently signed arms-control agreement. Rep. Jack Kemp (R-N.Y.) incessantly plugged free markets and "democratic capitalism" and admonished the others for talk of tax increases and cuts in Social Security cost-of-living increases.

And former secretary of state Alexander M. Haig Jr., in a pointed criticism of President Reagan, whom he served, said the next president must demonstrate leadership that is "more than just feeling good, but doing good, and we haven't been doing good enough."

Dole demonstrated his newly aggressive style when he was questioned about government ethics and the resignation of his campaign finance cochairman, whose handling of Dole family private finances has come under scrutiny.

Dole used the opportunity to recall that Bush, during the GOP debate in Des Moines last week, had demanded that all the candidates release their income tax returns for the last 10 years.

"I've done that today, George," Dole said. "Not just for 10 years but for 21 years. So I called you and I raised you."

"You did," a smiling Bush replied. "That's good."

Dole, who said he favored extending the Ethics in Government Act to members of Congress, complained that the controversy over the activities of David Owen, his longtime friend and former campaign finance cochairman, began when Bush aides distributed news reports questioning Owen's activities to reporters in Iowa.

In a reply that was typical of Bush's demeanor as he sought to stay above the battle, the vice president quickly agreed. "Sure they did," he said. "There was nothing clandestine about it."

One of the liveliest exchanges, involving all the candidates, came on the topic of population growth and the future of the Social Security system. Dole again took the first opportunity to go after Bush, including a jab at Bush's assertions that he was never fully aware of details of the Iran arms sales.

Discussing 1983 legislation that shored up the financial base of Social Security through an increase in the payroll tax, Dole held up a Bush campaign brouchure and said:

"I thought I fixed it, but George Bush says he fixed it, too. And I don't recall George being in the loop then, too. He takes credit for a lot of things."

"I was almost ready to vote for you after I read that," Dole said in a crack toward Bush.

"A lot of people will," Bush replied.

The debate fireworks were not confined to the two leading GOP contenders. At one point during the freewheeling session, Chancellor told the audience, "I'm losing control."

Chancellor later told the television audience, "I hope you didn't expect 'Rebecca of Sunnybrook Farm.' "

Du Pont, pushing his plan for a private alternative to Social Security, said the retirement system remained in danger of collapse early in the next century. Kemp added that Bush and Dole were "both wrong" on Social Security.

Dole accused Kemp of trying "sort of to have it both ways" on whether the controversial Seabrook nuclear plant should be opened, while Kemp attacked Dole's support for an oil import fee, an unpopular stand in this cold-weather state.

Several candidates ridiculed du Pont's proposal that teen-agers who fail to pass a drug test be denied a driver's license as a deterrent to further drug abuse.

"What if they don't have a car?" Dole asked sarcastically.

Haig said du Pont's idea sounded fine "on the campaign trail," but that the drug problem is concentrated in the nation's inner cities where technicalities like a driver's license don't matter much.

"I'm worried about a druggie stealing my car; he doesn't need a driver's license," Haig said.

Challenged by Chancellor to specify what government programs would be cut, du Pont said he would phase out farm subsidies and "stop subsidizing grain sales to communist countries, which George and Bob are for." Others ducked the question by endorsing the line-item veto or calling for adjustments in Pentagon procurement practices.

This prompted the relentlessly optimistic Kemp to complain that "the Republican Party is beginning to sound like the Herbert Hoover party," repeating his assertion that "fiscal restraint and lower taxes" would lead to a balanced budget.

The Republicans took a detour in their months-long debate over the arms control treaty that Reagan considers his biggest foreign policy success. Before the treaty was signed in December, they argued over its military and diplomatic impact, but today there was dispute over its text.

Robertson said the treaty does not reduce warheads because Soviet warheads would not be destroyed, but could be removed and transferred to more modern missiles. He read a transcript of the Oct. 28 Houston debate that quoted Bush as saying the treaty would eliminate 1,600 Soviet warheads for 400 U.S. warheads. "Now, George Bush, when the vice president of the United States doesn't know the difference between a missile and a warhead, we are in some serious danger," Robertson said.

Bush said he meant "bombs" and Robertson shot back "No, not bombs! No bombs!" Bush explained that he was talking about multiple-warhead missiles. Haig endorsed Robertson's position that the warheads could be adapted to more modern missiles. The treaty, while not requiring destruction of weapons' nuclear material, reduces the number of missiles carrying nuclear warheads in Europe and Asia.

Bush also sought to distinguish himself from the others by saying he would not require linkage between progress on arms control and on other U.S.-Soviet differences. "I wouldn't be afraid to give peace a chance," he said.

The Iran-contra affair, which has been a problematic issue for Bush in recent months, did not dominate the debate. Chancellor, referring to Bush's claim that he never heard objections from other senior Cabinet members to the Iran arms sales, asked the vice president, "It's very hard to believe you never talked to them and they never talked to you. What kind of policy management is that? Would you run that kind of White House?"

"No," Bush responded, saying the president tried to keep information about the arms sales "in highly compartmented modes" and corrections have been made in procedures. Bush also noted that former White House chief of staff Donald T. Regan had corroborated this account and Rudman had said Bush was correct in not revealing his advice to Reagan.