"It's clear that eliminating Saturday delivery would hurts tens of millions of Americans and countless small businesses while not addressing the financial problems," Rolando said in an e-mailed statement.
Donahoe has asked Congress to restructure or end that requirement, while seeking to close hundreds of post offices and mail-handling plants and to pull postal workers out of the U.S. government employees' health plan.
Lawmakers opposing cuts say they're voicing constituents' sentiments. Jon Tester of Montana, who received $65,000 in postal union donations, fourth most among Senate Democrats, on Wednesday called the Postal Service "absolutely critical" to the rural areas he represents.
Postal workers, of which there are 521,000, are among those constituents. The Postal Service's work force is larger than that of any publicly traded U.S.-based company other than Wal-Mart Stores.
Jeanette Dwyer, president of the National Rural Letter Carriers Association, said the contributions help the union motivate its members to lobby their representatives. "It's a grassroots tool," she said.
The postal unions are also mobilizing allies including MoveOn.org, which started a petition drive demanding Congress prevent closings of small-town post offices.
"The union money is less important than the fact that there are union members everywhere," said Art Sackler, coordinator of the Coalition for a 21st Century Postal Service, whose members include Bank of America and eBay and which supports cutting postal costs. "And there are relatives of union members everywhere. And they all care."
Still, Issa said, cutbacks are inevitable.
"We're going to ultimately all get to a numerically smaller Postal Service," he said.