Leaders of the nation's biggest postal union have been ordered to call a nationwide strike beginning as early as September if rank-arm-file workers reject a tentative contract, and the U.S. Postal Service fails to return promptly to the barganing table.

Delegates to the American Postal Workders Union convention here voted yesterday to require their offices to begin an illegal, national work stoppage if the union's 220,000 members fails to ratify the multibillion-dollar wage package. Mail balloting is taking place now. Results of the referendum will not be known until at least Aug. 23.

If the members accept the 19.5 percent pay raise contained in the proposed three-year contract, the threat of a nationwide mail strike from this union will virtually disappear. Although delegates here have rejected the contract by a 6-to-1 margin, the final say on the agreement - covering 550,000 unionized postal workers - is in the hands of individual members belonging to this union and the National Association of Letter Carriers and the Mailhandlers Union.

If APWU members reject the contract, the convention deadline gives the Postal Service five days to return to the bargaining table. And it sets a 15 day limit for "satisfactory" progress and improvement of the contract.

If the agreement is not improved susbtantially within 15 days, the 3,000 delegates here have ordered their leaders to call a strike. Although the action is binding only on this union, largest of the postal organizations, it would certainly trigger sympathy walkouts by members of other AFL-CIO unions.

Yesterday's session also approved financial and legal backing to workers - an estimated 180 - who have been fired by the Postal Service for wild-cat walkouts late last month. But the delegates carefully stopped short of laying everyone's job on the line. They voted heavily against a resolution that would have put all union members out on strike unless amnesty is granted to the fired workers.

Under federal law, strikes against the government are punishable by dismissal, a jail term and a $1,000 fine. Most of the fired workers worked in bulk mail centers handling foreign mail or domestic packages in New York, New Jersey and near San Francisco. A number of them are here at this convention. Some are guests, others delegates.

All are pleading for union backing to help them return to work, to raise funds for court fights, and to meet everyday living expenses. They were successful in the latter, but the delegates failed to answer the call for a nationwide strike unless amnesty were granted.

Many of the local union leaders are sympathetic to the plight of the fired workers. They blame the Postal Service for provoking the strike action in order to make examples of the relatively small group of workers who were dismissed. Many of the fired workers contend that their unions have worked with the Postal Service in their ouster, as a way of punishing "troublemakers."

But more of the delegates obviously are not willing to shut down the Postal Service and risk their own jobs by backing what they feel was an unwise, hastily made decision to wildcat before everyone had voted on the contract.

Yesterday's meeting was as calm and orderly as the Monday session was turbulent. On Monday a sizable group of delegates - about one-fourth of those present - disrupted the convention for 77 minutes, refusing to let President Emmet Andrews, speak, and calling for his resignations. Andrews, along with leaders of the two other AFL-CIO unions, signed the contract, which, in dollar terms, is below the expectations of most employes.

But Andrews pointed out that the contract has a job-protecting, no-layoff clause. He said that under the circumstances - with other federal pay frozen - "it was the best we could get."

Jimmy Carter was the president who took his lumps at yesterday's session. The union unanimously adopted a resolution asking the Democratic National Committee to keep Carter off the 1980 ticket if it wants the votes of postal workers and their families. Union officials estimate that bloc, counting husbands, wives and of-age children, amounts to 5 million people.

The union voted to pay $50,000 for the legal and living expenses of fired workers. And a pass-the-hat collection on the floor netted more than $8,400 in a short time. There were also substantial pledges from locals - including $50 and $100 donations from Washington, Maryland and Virginia groups - to help the fired workers.

One of the ex-employes, Sean Gordon, who identified himself as a Mailhander's union local leader from San Francisco, said he had spent $1,500 of his own money to come to this convention to help whip up support for amnesty.

Gordon said he didn't expect the convention to make amnesty a condition for avoiding a strike, and said he was "very happy" with the speeches calling for solidarity and especially for the dollar contributions to help the strikers.