Two Democratic lawmakers scolded the U.S. Postal Service on Monday for failing to explain how ending Saturday mail delivery would save billions of dollars each year as the agency claims it can do.

Sen. Claire McCaskill (D-Mo.) and Rep. Gerry Connolly (D-Va.) noted in a joint statement that they asked Postmaster General Patrick R. Donahoe in a letter six weeks ago to justify the postal service’s projected savings.

“The Postal Service has failed to deliver in responding to congressional requests in a timely and responsive manner,” Connolly said. “The postmaster general demands that Congress quickly rubberstamp the elimination of Saturday mail delivery — yet refuses to explain how cost-savings were calculated — apparently expecting members to evaluate his policy proposal from press materials alone.”

Donahoe last month announced plans to end Saturday mail delivery in an effort to save what he estimated would be $2 billion annually. The Postal Service said it lost nearly $16 billion during the last fiscal cycle.

Congress for the past several years has passed legislation prohibiting the Postal Service from dropping below six days of delivery. Donahoe’s move amounted to a gamble that lawmakers would not pass a similar measure this year.

A House proposal to fund the government through the end of September would require six-day delivery for the same period. The Senate is making amendments to the bill and is expected to approve the legislation this week.

The Postal Regulatory Commission wrote to McCaskill and Connolly last Thursday saying it could not calculate the savings from moving to a five-day delivery model without updated data from the Postal Service.

“It’s not responsible to move forward with this drastic plan without knowing if it would even make a substantial difference in dollars saved,” McCaskill said. “The Postal Service has the data we need, and it’s time for officials there to stop dragging their feet.”

Postal Service spokesman David Partenheimer said on Monday that attorneys for the Postal Service have met with staff members for the appropriate Senate and House committees and that the agency has provided a legal brief to the chairs and ranking members of those panels. He also said the Government Accountability Office is evaluating the cost savings of five-day delivery proposal.

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