Sen. Susan Collins, R-Maine, said she was disappointed, questioned the savings estimate and worried the loss of Saturday service might drive customers away.
"The Postal Service is the linchpin of a $1 trillion mailing and mail-related industry that employs more than 8 million Americans in fields as diverse as direct mail, printing, catalog companies, magazine and newspaper publishing and paper manufacturing," she said. "A healthy Postal Service is not just important to postal customers but also to our national economy."
She noted that the Senate last year passed a bill that would have stopped the postal service from eliminating Saturday service for at least two years and required it to try two years of aggressive cost cutting instead.
The House didn't pass a bill.
Republican House Speaker John Boehner said Wednesday, "I think trying to act in this postal area is pretty difficult. But I understand where the postal commission is coming from. They're in charge with running the post office, but yet the Congress, in its wisdom, has tied their hands every which way in order for them to actually run the post office in a revenue neutral way."
"And so Congress needs to act, there's no question about that, and I hope we'll act soon."
President Barack Obama's spokesman, Jay Carney, said the White House learned only Tuesday about the agency's decision to cut Saturday service. He said the White House is still evaluating the decision but would have preferred its own comprehensive overhaul package that failed to pass Congress last year be adopted "for the sake of a stronger future Postal Service."
The president of the National Association of Letter Carriers, Fredric Rolando, said the cutback is "a disastrous idea that would have a profoundly negative effect on the Postal Service and on millions of customers," particularly businesses, rural communities, the elderly, the disabled and others who depend on Saturday delivery for commerce and communication.
He said the maneuver by Donahoe to make the change "flouts the will of Congress, as expressed annually over the past 30 years in legislation that mandates six-day delivery."
The National Farmers Union said "impacts on rural America will be particularly harmful."
Despite that opposition, the Postal Service clearly thinks it has a majority of the American public on its side. The service's market research indicates that nearly 7 in 10 people support the switch as a way to reduce costs, Donahoe said.
He said the savings would include employee reassignment and attrition.
The agency in November reported a record annual loss of $15.9 billion for the past budget year and forecast more red ink in 2013, capping a tumultuous year in which it was forced to default on the $11 billion in retiree health benefit prepayments to avert bankruptcy.
The financial losses for the fiscal year ending Sept. 30 were more than triple the $5.1 billion loss in the previous year. Having reached its borrowing limit, the mail agency is operating with little cash on hand.
The Postal Service is in the midst of a major restructuring throughout its retail, delivery and mail processing operations. Since 2006, it has cut annual costs by about $15 billion, reduced the size of its career workforce by 193,000, or 28 percent, and has consolidated more than 200 mail processing locations, officials say.
At the DeCicco Food Market in Pelham, N.Y., where they handle the mail for all 10 of their stores, the assistant manager, Frank Torres, said they will try to adjust their routine. I'll tell you, though, the customers are pretty upset, we've been hearing them at the (checkout) registers. Some of them get mail they want on Saturday.
"You know what also, it will be strange not seeing the mailman on Saturday. Our office girls know him on a first-name basis."
Associated Press writers David Koenig in Dallas; Jim Fitzgerald in White Plains, N.Y.; John Seewer in Toledo, Ohio; Nedra Pickler and Jerry Bodlander and researcher Monika Mathur in Washington contributed to this report.