Timeline: Decades of Delivering Mail

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But this time the message is being delivered as the Postal Service looks to stem mounting losses that last year neared $16 billion, and a few Senate Republicans are pushing for a change to the spending bill that they say would give the Post Office the leeway it wants.

"Congress will be hamstringing the Postal Service, hastening its demise and probably adding additional financial burdens to U.S. taxpayers" if it requires six-day-a-week service, said Sen. Bob Corker (R., Tenn.). Mr. Corker, along with Sen. Tom Coburn (R., Okla.), has offered an amendment to drop the six-day service requirement.

The Postal Service, which operates as an independent government agency but is also subject to the oversight of Congress, earlier this year said it would in August end most weekend deliveries but still ship packages. The move could save as much as $2 billion a year at an agency that lost $15.9 billion last year, causing it to tap out its credit line from the U.S. Treasury. The losses are expected to continue unless Congress allows changes to retiree obligations and health-care expenses, in addition to limiting Saturday service.

While the amendment appears unlikely to gain approval as congressional leaders try to keep the spending bill on track for final passage before March 27, when the current government spending provisions expire, it opens the door for debate. There is disagreement in Congress on whether legislation stops the Postal Service from cutting Saturday services.

"Nothing in the language passed by the House prevents them from doing that," said Rep. Darrell Issa (R., Calif.). Mr. Issa said he believes that as long as Postal Service delivers medicines, Internet orders and other packages on Saturday, it is meeting the mandate for six-day service.

Other lawmakers said Saturday delivery must continue. "The language is clear," said Rep. Jose Serrano (D., N.Y.). "They can't put these changes to Saturday mail into effect."

Ruth Goldway, chairman of the Postal Regulatory Commission that oversees postal operations, said the provision in the spending bill would make it hard for the Post Office to implement the Saturday cuts.

"With language in the appropriation bill for over 30 years, the consensus is that the Postal Service must deliver mail on Saturdays," Ms. Goldway said. "If the language stays, I'd be surprised if that consensus changes."

The Postal Service, meanwhile, said it continues to plan for a cutback in services, setting up a possible showdown with Congress later this year. The Postal Service "will continue its planning for the August implementation of the new delivery schedule, while Congress debates," spokesman Dave Partenheimer said Friday.