A bipartisan group of four Senators is hoping to pressure House Republicans to pass U.S. Postal Service reform legislation before May 15 when a moratorium on closing postal facilities expires, but the foursome is also urging the Postmaster General to extend the moratorium.

Sen. Tom Carper (D-Del.) today launched a countdown clock on his website warning that the closure of more than 1,000 Postal Service facilities could begin if the House does not act before the deadline.

“My friends in the House must move forward with consideration of postal reform legislation right away or else we’ll begin to see the dire consequences of postal facilities shutting down across the country,” Carper said in a release announcing the clock. “Failing to act would hasten the Postal Service’s financial decline, which would threaten a mailing industry that employs over 8 million people and generates almost $1 trillion in economic activity each year.”

“We can’t let that happen,” Carper continued. “If Congress works together to finalize comprehensive postal reform legislation, we can ensure that this invaluable American institution, enshrined in the Constitution, will survive and thrive in a new century. But the clock is ticking.”

Postmaster General Patrick Donahoe agreed not to close any facilities until May 15 as Congress worked out postal reform legislation. The Senate passed a measure before the recess that would allow for the orderly closure of postal facilities and slowly scale back service, as well as reduce the Postal Service workforce. But the House has yet to move a companion measure. Without House action, Carper warned, there could be an abrupt and economically damaging closure of facilities across the country.

The Senate reform measure is designed to help the Postal Service get its finances in order. The Postal Service, with about 570,000 employees, is the second largest civilian employer after Walmart. But with the increase in paperless communication and a commensurate decline in mail volume, the agency projects a $14.1 billion net loss for fiscal 2012.

Carper is part of a bipartisan group of four Senators, including Homeland Security and Governmental Affairs Chairman Joe Lieberman (I-Conn.) and ranking member Susan Collins (R-Maine). Carper is the chairman of the panel’s Subcommittee on Federal Financial Management and Sen. Scott Brown (R-Mass.) is the ranking member.

The four wrote a letter dated Monday to Speaker John Boehner (R-Ohio) and House Minority Leader Nancy Pelosi (D-Calif.) urging the House to follow the Senate’s lead and pass a similar measure.

“The Senate last week passed a bipartisan postal reform bill designed to preserve the United States Postal Service and put it on a solid financial ground for the next generation of Americans,” the letter said. “We are writing to urge that the House of Representatives act promptly, so that the two chambers can reconcile their bills to turn around the Postal Service’s daily loss of $25 million, prevent the unnecessary wholesale closing of regional mail facilities and local post offices, and save this iconic institution that delivers over 500 million pieces of mail a day and sustains over 8 million jobs.”

The House is expected to move its bill before the summer recess. The House measure, written by Oversight and Government Reform Chairman Darrell Issa (R-Calif.), differs from the Senate measure in that it would establish a control board to implement cost cutting initiatives and quickly downsize the Postal Service.

“We believe the Senate bill … takes a better approach,” the letter said. “Under the Senate bill, the Postal Service would use the same kinds of retirement-incentives that private industry uses to restructure and voluntarily ‘right-size’ its workforce, to reduce the postal workforce over a period of 3 years by 18 percent, or roughly 100,000 positions, eventually saving $8 billion per year. The bill also establishes an orderly and predictable process for achieving a more optimal network of post offices and mail processing plants, requiring involvement of local communities to ensure that essential postal services are preserved.”

Issa and other opponents of the Senate bill have said that the measure merely delays the difficult decisions that must be made to make Postal Service fiscally sustainable.

Today from his Twitter account, Issa pointed to an editorial in USA Today that argued that the Senate bill “ducked some of these unpopular steps.”

“Nothing pleasant about … closing post offices. But alternative is forcing taxpayers to subsidize a bloated system that operates at a loss,” Issa said via Twitter.

Meanwhile, the four Senators wrote to Donahoe, urging him to extend the moratorium and give Congress more time to reform the Postal Service.

“We look forward to working with the House of Representatives and the Postal Service to enact final legislation in the coming months,” the Senators said. “We are writing to ask that, in the interim, you follow the intent of the Senate with respect to the closures of post offices and processing facilities and that you extend the current moratorium to cover closures that would not be permitted under [the Senate bill] until Congress has an opportunity to finalize legislation in the near future.”