Consumer activist Ralph Nader offered yesterday to join his old nemesis--General Motors Corp.--in a television commercial aiding the administration's campaign to promote the use of auto seat belts.

But it appears that General Motors has turned their chief adversary down. In a brief statement issued last night by the head of the public relations staff, John W. McNulty, General Motors said it was already engaged "in a serious effort to promote seatbelt usage," developing newpaper, magazine and radio promotions to explain the benefits of seat belts. "This is a serious matter and not a time for a personality contest," McNulty added.

Through the National Highway Traffic Safety Administration Nader had offered to join GM Chairman Roger Smith in an ad that would be a part of the administration's $7 million public relations campaign to promote the use of seat belts. The campaign was announced by President Reagan at a White House ceremony Wednesday.

In a telephone interview, Nader said he doesn't think the ad campaign will work, "but if it saves one life, it's worth doing." He said his offer to appear with Smith was "my way of testing" to see how serious GM is about encouraging the use of seat belts.

As envisioned by Nader, the 30-second public service ad would have him and Smith seated in the front seat of a car. "I say, 'I don't agree much with GM,' and he says, 'I don't agree with Ralph Nader.' But we say we both say we agree on one thing: As long as you have your seat belts, why not use them?"

Nader said he first made the offer four years ago to Pete Estes, then GM's president, but Estes rejected it.

"I don't think GM is serious about seat belts and this is my way of testing them," Nader said.