The United Auto Workers had to agree to early contract talks with car makers to avoid the possibility of a strike--"a trap" that would hurt the union--when the current agreement expires in September, UAW President Douglas A. Fraser said last night.

Right now, the union has something the General Motors Corp. and Ford Motor Co. need--the power to grant an early settlement with wage and benefit concessions that can save the companies millions of dollars in labor costs before the contract expires, Fraser said.

But if there is no agreement in place by Sept. 15, the union loses what leverage it has, and the companies would then have the power to "unilaterally demand economic concessions," Fraser told several hundred delegates here at the opening of the UAW's seventh annual political action conference.

"That would leave us with the Hobson's choice" of giving back more than the union is willing to concede "or going on strike . . . . And if they challenge us to strike, it's going to be a long and bitter strike," Fraser said.

He implied that for the union, already hit with 228,00 workers on indefinite layoff since Jan. 18, it would also be a losing strike. Fraser said January marks the 25th month of "depressed domestic auto sales," which means the companies have huge inventories. Huge inventories mean the companies can afford to take a strike better than the union can afford job losses that might result from any walkout.

"Collective bargaining is a matter of timing," Fraser said in comments designed to shore up support for the UAW leadership's decision to resume emergency talks with GM and Ford. The talks began Jan. 10, but broke off last Wednesday in disputes over wage and benefit concessions, job security provisions, and, in GM's case, the implementation of an agreement to allow consumers to benefit from labor cost savings.

The union's GM and Ford councils agreed Saturday to return to the bargaining table. Talks resume Tuesday with GM--the focal point of the negotiations--and on Friday with Ford.

Fraser has set a deadline of midnight Thursday to reach an agreement with GM. But his comments last night seemed to imply that he would be willing to go beyond that time if a settlement is in sight.

Fraser said that GM's sales have virtually stopped since the company promised two weeks ago to allow wage and benefit concessions be reflected in lower sticker prices on its cars and trucks. Rebate hungry consumers won't come back until an agreement is in place, which puts extra pressure on the company to come up with an emergency agreement that will save money and protect union jobs, Fraser said.

Ford has no agreement to pass on labor costs savings to consumers. But analysts agree Ford will have to come up with a similar plan to remain competitive with GM.

It all means that at the moment, despite its weakened position, the UAW has some clout. "But our bargaining power will be greatly diminished in September, 1982," Fraser said.