CONCORD, N.H., FEB. 2 -- The six Republican presidential candidates briefly put aside their campaign differences tonight and pledged their unqualified opposition to gun control laws and their devotion to the Second Amendment's guarantee of "the right to bear arms."

Campaigning across the state today before most of them head west for the first showdown of the 1988 campaign in the Iowa caucuses next week, the GOP contenders found themselves in harmony before an approving audience, the Gun Owners of New Hampshire.

"My idea of gun control is a steady aim," said Rep. Jack Kemp (R-N.Y.).

None of the Democratic candidates was present, although Sen. Albert Gore Jr. (D-Tenn.) had originally accepted. All of the Democrats were booed as their names were called out, with the most vigorous booing directed at Gov. Michael S. Dukakis of neighboring Massachusetts.

Vice President Bush was the only Republican to be greeted by a scattering of boos, but he recouped when he recounted registering his firearms in Washington under the District of Columbia's gun control law.

"I asked if any high-profile criminals had come in to register their guns and I was told, 'No, just suckers like you from Northwest Washington,' " Bush said to applause.

"Instead of going after firearms, our administration has gone after criminals," Bush added.

Earlier in the day at a state legislative committee hearing on abortion legislation, former television evangelist Pat Robertson attacked Planned Parenthood, whose federal funds he said he would cut off if elected.

"Back in the 1920s, Margaret Sanger, one of the founders of Planned Parenthood, wrote a book on eugenics entitled 'Breeding the Thoroughbred,' " Robertson said. "They wanted a master race. They advocated sterilization of blacks and other minorities, of Jews and fundamentalist Christians."

{Planned Parenthood President Faye Wattleton in New York called Robertson's remarks "unfounded and, frankly, ridiculous," the Associated Press reported.}

At the gun owners' forum, the only semblance of potential disagreement involved pending legislation aimed at weapons made of plastic or other materials that are undetectable by existing security devices. Kemp, Bush, Sen. Robert J. Dole (R-Kan.) and former secretary of state Alexander M. Haig Jr. each cautioned that compromise may be necessary in the future to balance the rights of gun owners with the need to protect travelers and police officers.

Pulling from his pocket a small .22-cal. pistol that he said was undetectable, Bush said, "This can kill the pilot of an airplane. I don't want the police and Secret Service on the opposite side of gun owners."

However, all the candidates, who also included former Delaware governor Pierre S. (Pete) du Pont IV and Robertson, promised they would veto the pending legislation.

With no disagreements on gun control, the GOP aspirants emphasized their conservative credentials, calling for continued aid to the Nicaraguan contras and closing with excerpts from their stump speeches.

The appearance before the gun enthusiasts was the climax of a last day of heavy campaigning here before the state, which holds its primary Feb. 16, is virtually depopulated of presidential candidates until after Monday's caucuses in Iowa.

Earlier, Dole continued to ridicule Bush's lack of a record of "leadership" and directed his heaviest fire at Kemp, whom he accused of distributing "political trash" that "distorted" his record on Social Security.

Dole, claiming to be the only GOP candidate to outline a plan to reduce the federal budget deficit, also challenged his rivals to do the same.

"I'm still waiting to hear George Bush's plan on the deficit," he said after discussing the environment in a speech at St. Paul's School here.

"He says he's going to reduce the capital gains {tax} rate to 15 percent. That costs $17 billion a year. He's going to have a savings plan for education. Is that it?"

Kemp, who has risen to a strong third place behind second-place Dole and front-runner Bush, again drew fire for a mailing to elderly voters that used envelopes promising "important Social Security information" inside. The mailing, which was attacked earlier when it was used in Iowa, accuses Bush and Dole of planning to reduce "future Social Security benefits" because of their support for a failed 1985 budget measure that would have frozen cost-of-living adjustments in several programs, including Social Security.

Dole called the mailing "deceptive" and "on the borderline of fair campaigning." He also accused Kemp of spreading "messages of fear to senior citizens" without telling them that Kemp opposed 1982 legislation that Dole claims rescued Social Security from insolvency. The Dole campaign also called in its most prominent New Hampshire supporter, Sen. Warren B. Rudman (R), who said that Kemp was engaging in "the politics of fear and intimidation of a very vulnerable group" and that such tactics "never work in New Hampshire."

{"I think my record is very strong with regard to support of Social Security and . . . is even stronger when you compare it against du Pont and Robertson, who want to privatize it, and Dole and Bush, who have both voted to freeze it," the AP reported Kemp as saying in Manchester, N.H.}

Staff writer David Hoffman contributed to this report.