The plan to cut delivery of letter mail while retaining package delivery on Saturdays "rests upon a faulty USPS premise," GAO General Counsel Susan Poling said in the letter.
The service, after losing $15.9 billion last year and reaching its legal borrowing limit, said last month it plans to eliminate a day of mail delivery to save about $2 billion a year.
David Partenheimer, a Postal Service spokesman, had no immediate comment.
Connolly and Sen. Claire McCaskill, D-Mo., also asked the Postal Regulatory Commission, which oversees rates and service standards, for an opinion on whether cutting Saturday delivery is allowed.
"This impartial and definitive GAO legal opinion makes it crystal clear that USPS cannot operate outside the legislative authority of Congress and unilaterally implement a change in delivery service that many believe will not only disrupt mail service, but also exacerbate USPS revenue losses and contribute to the decline of this constitutionally-mandated service to all Americans," Connolly said in an emailed statement.
Lawmakers have authority over the Postal Service because it receives an appropriation for less than 1 percent of its operations. That money includes reimbursing the service for mail for the blind and for election ballots for overseas military personnel.
Connolly and McCaskill were among Democrats, also including Senate Majority Leader Harry Reid of Nevada, who criticized the service's Feb. 6 announcement that it plans to end Saturday delivery in August while it waits for Congress to pass a comprehensive measure to overhaul its business model.
Rep. Darrell Issa, R-Calif., and Sen. Tom Coburn, R-Okla., praised the announcement last month and Thursday sent a letter to the Postal Service's board urging it to keep preparing to end Saturday delivery. They said the service isn't cutting a day of service, as it has said it will deliver packages on Saturdays.
"We believe that the Board of Governors has a fiduciary responsibility to utilize its legal authority to implement modified six-day mail delivery," the lawmakers wrote in the letter. "The deficits incurred by the Postal Service and the low level of liquidity under which it is operating leaves it in a perilous position, one that demands implementation of all corrective actions possible."
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