Patrick R. Donahoe, postmaster general and CEO, said at a press conference that the move is "absolutely necessary" as part of a broader effort to stabilize the service's troubled finances.
"Making this change to our delivery schedule is a big-ticket item," he said. "It would be irresponsible for the Postal Service not to pursue this course."

Under the proposal, the Postal Service will continue to deliver packages six days a week. The plan, which is aimed at saving about $2 billion, would start to take effect in August.

It's unclear, though, how the service can eliminate Saturday mail delivery without congressional approval.

For the past 30 years, Congress -- which oversees the otherwise independent agency -- has included a provision insisting on Saturday delivery. That provision still stands, leaving some on the Hill bewildered about the announcement Wednesday.

Fredric Rolando, president of the National Association of Letter Carriers, said Wednesday that the move "flouts the will of Congress."

"Postmaster General Patrick Donahoe's plan to end Saturday delivery is a disastrous idea that would have a profoundly negative effect on the Postal Service and on millions of customers," Rolando said in a statement. "It would be particularly harmful to small businesses, rural communities, the elderly, the disabled and others who depend on Saturday delivery for commerce and communication."

But Donahoe said "we think we're on good footing with this." He noted that the law governing the agency's operations expires at the end of March, and said if there's any disagreement it can be resolved by then.

The announcement garnered support from two Republicans on Capitol Hill, California Rep. Darrell Issa and Oklahoma Sen. Tom Coburn. They called the move a "common-sense reform" that would save money.

The agency clearly thinks it has a majority of the American public on its side regarding the change. Material prepared for the Wednesday press conference said Postal Service market research and other research has indicated that nearly 7 in 10 Americans support the switch to five-day delivery as a way for the Postal Service to reduce costs.

Donahoe noted Wednesday that the agency posted a nearly $16 billion loss last year. "By any measure, that is unsustainable and it's unacceptable," he said.

The agency previously looked at ending both mail and package delivery on Saturdays. The move to preserve weekend package delivery accentuates one of the agency's strong points -- package delivery has increased by 14 percent since 2010, officials say, while the delivery of letters and other mail has declined with the increasing use of email and other Internet use.

Under the new plan, mail would still be delivered to post office boxes on Saturdays. Post offices now open on Saturdays would remain open on Saturdays.

Over the past several years, the Postal Service has advocated shifting to a five-day delivery schedule for mail and packages -- and it repeatedly, but unsuccessfully, appealed to Congress to approve the move. Though an independent agency, the service gets no tax dollars for its day-to-day operations but is subject to congressional control.