The American Postal Workers Union, the nation's largest organization of postal employes, declared legislative war yesterday on President Reagan's plans to cut $632 million from the $1.5 billion federal postal subsidy for fiscal 1982.

In some quarters of the U.S. Postal Service, the "lobby fight" warning from APWU President Moe Biller was regarded as the opening shot in a collective bargaining battle that officially begins late next month.

Biller said yesterday that the proposed reduction "will cost 40,000 postal workers their jobs" and that the prospective loss leaves his 320,000-member union "no option but to fight back . . . to join the other postal unions, professional organizations and working men and women everywhere" to lobby against the president's program in Congress.

The union leader objected to Reagan's charge the groups opposing the administration's tax and spending cut proposals are being "selfish."

"I don't know what selfishness means. If he's talking about putting people out of work and adding to the welfare rolls, then those who continue to work are going to have to bear that burden," Biller said.

He added: "It isn't a matter of being selfish. We have living standards to maintain. . . So have all American workers."

Biller, who based his estimate of potential job losses on AFL-CIO research, reluctantly agreed that the budget-cut issue will play a major role in Postal Service collective bargaining that begins April 20. The negotiations between the service management and the APWU and smaller postal unions will affect 600,000 workers -- the largest number involved in contract talks this year.

After first saying that he "can't see an impact on the collective bargaining" because of the planned $632 million cut. Biller said: "Well, obviously, that type of cut has to make it difficult for everything. . . ."

"I don't want to negotiate in the press," Biller said. But, he continued:

"There is nothing that I deem more important and more necessary than the forthcoming negotiations. . . . These negotiations will be a very important test for the public worker and the entire concept of collective bargaining in the federal sector."

Postal Service management sources reached yesterday agreed. "These cuts are going to have to come to play in the negotiations. There's no way of getting around it," one source said.