The United Auto Workers today targeted the General Motors Corp. -- biggest, strongest and busiest of the nation's car makers -- for a possible strike next month.

But the union may strike selected plants, rather than trying to close down the whole company if its Sept. 14 strike deadline passes without a new contract, UAW President Douglas A. Fraser later told reporters.

A strike against GM, the nation's biggest industrial firm with $63.2 billion in sales last year and nearly 500,000 workers in 130 plants in 23 states, could send a major shudder through the nation's faltering economy.

In an apparent departure for the union, Fraser indicated the UAW may employ selective strikes where it would hurt GM the most -- against plants producing the company's popular new small cars -- while still protecting the union's $290 million strike fund and keeping more members at work.

"It is possible there will not be a national, total strike . . . we're not ruling out any strategy," said Fraser, adding that the union's main effort will be to exert "maximum pressure in the shortest period of time with minimum sacrifice."

Normally the 1.5 million member UAW closes down a target company if it fails to reach a new contract by the strike deadline. In 1970, the last time GM was targeted, the company was shut down 67 days in the nation's costliest labor walkout.

GM had been expected by many observers to be the target again this year because the union targeted the Ford Motor Co. during the last round of auto negotiations in 1976 and had already ruled out the Chrysler Corp. because of severe financial problems. But some thought the size and strength of GM -- and the cost of striking it -- might give the union second thoughts.

It did not, according to Fraser. He said GM's fast-moving sales and low inventories make it vulnerable to a shutdown. He noted that, as of July 31, GM had 20 to 25 car models with inventories of less than 60 days while Ford had only three models with inventories that size.

Any selective strikes are expected to be aimed at the swiftest-selling cars, mainly the new front-wheel-drive compacts and other small fuel-efficient models. It would be "suicidal" to strike any plants producing models with heavy inventories, he noted.

Both Fraser and Irving Bluestone, UAW vice president for General Motors, said they hoped the union could settle without a strike -- a possibility that appeared to be heightened by the union's moderate response to initial contract offers earlier this week from GM and Ford.

The three-year contract offers, which Fraser rejected but described as the "foundation upon which we can build . . . a satisfactory agreement," call for continuation of the union's historic 3 percent annual wage increases and its existing cost-of-living protections, along with an increase of 23 percent in pension benefits for new retirees over three years.

But the union is also seeking cost-of-living protections for retired employes, which the companies did not propose, and a number of other contract improvements still in dispute.

It will be close to the Sept. 14 bargaining deadline before even the union can tell whether a strike is likely, Fraser added.

The last time the UAW negotiated a new auto industry contract without a strike was in 1964.

Although both the union's demands and the initial GM and Ford offers appear to exceed the Carter administration's anti-inflation guideline of 7 percent a year for wage and benefit increases, the administration has kept hands off the auto talks.

At one point Fraser told the White House to "stay the hell out" of the negotiations, and a GM source said today the wage guideline has not come up in the talks.

Fraser had suggested earlier that a one-year contract -- instead of the normal three-year pact -- might be necessary in light of continuing inflation and the administration's short term wage-containment efforts.

He did not bring up the idea today, although, when asked, he said it is "still an open question" that hinges on the substance of the agreement. He said he would prefer a three-year contract, and GM's offer was for three years.

The decision to target GM was made by Fraser and the union's 26-member executive board and was cheered loudly by bargaining councils for GM and Ford workers who met shortly after the executive board's meeting to hear the results.

Although there was what Fraser called "some difficult discussion" about the choice by the executive board, he said the union is "of one mind now."

Fraser said UAW and GM negotiators will begin intensive bargaining Friday morning and continue through the Labor Day weekend.

Geroge B. Morris, GM vice president for industrial relations, said the company "is prepared to enter intensive negotiations with the UAW aimed at reaching a contract settlement before the Sept. 14 strike deadline."

He said the company's rejected first offer "provides a sound basis for resolving the issues" and asserted that auto workers' wages and benefits "far exceed those received by most American workers." GM calculates that the average auto worker receives $15.10 an hour in wages and benefits.