Sens. Joseph I. Lieberman (I-Conn.) and Thomas Carper (D-Del.) announce a bill to overhaul the U.S. Postal Service, in November, on Capitol Hill. (Andrew Harrer/BLOOMBERG)

A bill co-sponsored by Sens. Joseph I. Lieberman (I-Conn.), Thomas R. Carper (D-Del.), Susan Collins (R-Maine) and Scott Brown (R-Mass.) has emerged as the leading Senate proposal to confront a series of financial and structural challenges facing the cash-strapped delivery service. It is expected to once again lose billions of dollars this year, mostly due to unique prepayments it must make to fund future worker health benefits.

The bill would permit the end of Saturday mail deliveries and the closure of thousands of post offices and processing centers. It also would overhaul the system’s finances and give it back billions of dollars it’s paid into federal retirement accounts.

Sen. Bernard Sanders (I-Vt.). (AP)

But in a letter sent Tuesday, 27 Senate Democrats asked for “significant improvements” to the bill. The same coalition, led by Sen. Bernie Sanders (I-Vt.), successfully convinced USPS to wait until at least May 15 to begin closing smaller, less utilized post offices across the country.

Here’s what the group is asking for:

— Safeguards for rural post offices: “It makes no economic sense to have hundreds of people spending hours driving back and forth multiple times to a distant post office rather than maintaining a local post office, potentially with reduced hours,” the senators wrote. USPS is planning to close thousands of tiny, underutilized post offices, mostly in rural and densely urban communities. But Sanders, whose home state is dotted with picturesque post offices usually located in town centers, believes closing the rural locations would cause a significant inconvenience.

— Maintain current delivery standards: The Postal Service is exploring the feasibility of closing up to 253 mail processing facilities across the country, a move that would no longer permit USPS to guarantee overnight delivery of some first-class mail. “USPS cannot afford to disappoint its customers in this manner,” the senators wrote. “It does not exist in a vacuum but rather competes for market share with private services that have the capacity to offer convenient and expedient delivery.”

— Maintain Saturday mail delivery for four more years: The Lieberman-Carper-Collins-Brown bill would end Saturday deliveries after two years.

— Establish a “blue ribbon entrepreneurial commission” to explore ways for USPS to earn money: Virtually every postal bill in the House and Senate already proposes doing this. The Lieberman-Carper-Collins-Brown measure would allow USPS to explore partnering with city, state and federal agencies to sell or provide government services at post offices.

For his part, Carper — arguably the Senate's leading expert and advocate for postal reform — said last week that he appreciated Sanders’s concerns. But, his spokeswoman Emily Spain added: “Make no mistake, some sacrifices from Postal management, Postal employees and Postal customers will be necessary if we want to ensure that this important American institution that can survive.”

Follow Ed O’Keefe on Twitter: @edatpost

Further reading:

Obama’s 2013 budget backs ending Saturday mail, boosting stamp prices

Federal Diary: Government continues to shrink, despite ‘obesity problem’ rhetoric

For more, visit PostPolitics and The Fed Page.