After a week of sustained militancy, postal-worker delegates to a convention here who have sought rejection of a proposed 3-year contract, threatened a national mail strike, sharply critized their union leaders and demanded that President Carter resign because they said he is a liar, spent their last convention day bogged down on the question of where to meet in 1980.

The bitter battle for the convention city - with Detriot nosing out Honolulu and Los Angeles - would have struck a one day visitor as a petty, make-work exercise. But the long debate and vote on the issue was enmeshed in politics, as have been most things at the American Postal Workers Union meeting .

Most of the union's national leaders favored Honolulu as the next meeting site for this AFL-CIO union of 200,000 members. But delegates, arguing that Detriot is cheaper to get to and live in - and the mood of New York militants to go against anything the leadership wanted - tipped the balance in favor of Detriot.

Most actions taken by this convention have been caught up in a power struggle between incumbent president Emmet Andrews, and the leader of the big and vocal New York delegation, Moe Biller. Biller controlled one-third of the votes here. And he was features on Denver television interviews nearly as often as the national president.

Although Biller originally nominated Andrews for his job, he frequently has "repented" the action. His delegation led the demonstration Monday that blocked all business for nearly two hours, and the successful effort to demand rejection of the tentative contract owith the Postal Service.

APWU delegates, representing the largest bloc of the nation's postal workers, overwhelmingly asked rejection of the 3-year contract, which would include a 19.5 percent raise and a promise that there will be no further layoffs for any reason. But rank-and-file members will ultimately decide whether to accept or reject the contract.

On Monday, union members - bitter because they feel prounion candidate Carter, whom they backed, has become an antiunion president - wired the Democratic National Committee. They asked the DNC to drop Carter from the 1980 ticket. Later on, they sent a "special delivery" letter to the White House (with a $2 stamp) demanding that the president "live up to his pledge to resign if he ever lied to his American people."

Union members feel the White House has betrayed them by failing to push the Senate to amend the "no politics" Hatch Act. A change would allow them and other workers to take active roles in partisan politics. Although Carter has endorsed the bill, he has not pressured the Senate to vote on it, and demanded that the Hatch Act rider be removed from his pending civil service reform bill.

Workers also are upset because they feel the president illegally intruded into their contract talks, and failed to support legislation to pump more money into the sagging Postal Service.

The two biggest stars were visitors from Washington, Rep. Bill Clay (D-Mo.), author of the Hatch Act "reform" bill, delighted delegates when he told them to get active, get political and get tough because they had become the "new niggers of America". Clay made the same speech two weeks ago at the American Federation of Government Employees convention.

The other guest at the APWU convention was Rep. Charles H. Wilson (D-Calif.). He won an ovation when he said he will introduce legislation giving postal workers a right to strike "anytime your union leaders want me to."

It was probably safe to say that this convention ended on a low note. The bitterness and division were the worst in memory, some oldtimers said.

Union delegates were criticized by the Denver press, which printed charges by hotels complaining that rowdy night-time revelers - and there were some - had destroyed property and set off fire alarms. The union responded that the problems were caused by a relatively small number of people.

Many here blamed the convention hotels for adding to frustrations. Hotel construction blocked off main entrances, elevators were overcrowded, there were several telephone breakdowns and a scramble at check-in and check-out time.