WASHINGTON — All but five of Congress’s 255 Democrats and independents received campaign donations from postal worker union groups in the past six years, raising the political risk of Postmaster General Patrick Donahoe’s move to end Saturday mail delivery.

Political action committees for the seven postal unions contributed $9.6 million from 2007 to 2012 to current members of Congress, 91 percent of it to Democrats and two independents who caucus with them, according to data compiled by Bloomberg from the Federal Election Commission and the Center for Responsive Politics, a Washington-based research group.

Sen. Susan Collins, R-Maine, a co-sponsor last year of the legislation that would have required waiting at least two years to end Saturday mail delivery, received $70,500, more postal union donations than any Republican in Congress. Calls to Collins’s office over two days weren’t returned.

California Rep. Darrell Issa, chairman of the House committee with oversight over the Postal Service, had the most donations among House Republicans, $44,500. Issa, who has said he backs ending Saturday mail delivery, said the union donations don’t affect his position.

Democrats control the U.S. Senate, which must agree to most of the changes Donahoe says are needed to save the Postal Service from insolvency. Many of his proposals are intended to reduce labor costs accounting for 80 percent of the service’s expenses. That puts Donahoe in conflict with post office unions, which would lose most of the estimated 22,500 jobs that would be cut if Saturday delivery ends, and have spent years making friends on Capitol Hill.

“That’s why it’s been so hard to come up with a plan for the Postal Service,” said Bill Allison, editorial director of the Sunlight Foundation, a Washington-based watchdog group. “The obvious thing you want to do is cut back on the number of employees, cut back on services, cut back on benefits. That’s something Democrats haven’t wanted to do in part because of the support they’ve gotten from the unions.”

Donahoe is trying to cut $20 billion a year in costs after the Postal Service, in the face of declining mail volume due in part to email and online commerce, lost $15.9 billion last fiscal year and an additional $1.3 billion in the quarter that ended Dec. 31.

The postmaster general yesterday stood by his position that ending Saturday mail delivery in August is legal, even though appropriations bills for three decades have required six-day mail, with the government operating under temporary funding.

Senate Democrats including Majority Leader Harry Reid of Nevada insist they, not Donahoe, get to decide on ending Saturday delivery. The Senate passed a measure last year that gave Donahoe some of what he wanted, while blocking him from ending Saturday mail delivery for at least two years. The House didn’t vote on that measure or its own plan.

“It may be in the public interest to curtail Saturday mail delivery, but the decision may not be made in economic interests but in the interest of who their friends are,” said Gary Chaison, a labor professor at Clark University in Worcester, Mass.

Every elected member of the Senate Democratic caucus has received contributions from postal union political committees, the records show. Of the 55 Senate Democrats, only Brian Schatz of Hawaii and Mo Cowan of Massachusetts, who were appointed, haven’t received donations from at least one postal union PAC. By comparison, 19 of the Senate’s 45 Republican got postal contributions.

Stephen Lynch, a Democratic House member from Massachusetts running in the special election to fill new Secretary of State John Kerry’s Senate seat, received the most among House members in the past six years. Lynch, whose mother was a postal clerk and father was an ironworker, received $175,100 from postal PACs, the records show.

A campaign spokesman, Conor Yunits, said the lawmaker always has been close to postal and ironworkers unions because of his parents. “He’s very proud to have that support and these organizations have always been very important to his family,” Yunits said.

Sen. Chris Murphy, D-Conn., received $96,500 in contributions to lead all senators, though his figures also include donations to his House campaigns in 2008 and 2010. A spokesman, Ben Marter, said the contributions have no impact on how he votes. “Chris fights for jobs wherever they are — factory floors, schools, hospitals, or post offices,” Marter said.

In the House, 197 out of 200 Democrats received postal union PAC donations, compared with 72 of the 232 Republicans.

The biggest postal union donor was the National Association of Letter Carriers, which trailed only the American Federation of Teachers among union political committees in campaign giving for the 2012 elections.

Democrats say they’re concerned about the impact of postal service changes on employees. Maryland Rep. Elijah Cummings, the top Democrat on Issa’s House Oversight and Government Reform Committee, said 45 percent of Postal Service employees are minorities, 40 percent are women and 21 percent are veterans. Some might not be able to find other work if they lose their jobs, he said.

“Our main concern is compassion for those who have given their blood, sweat and tears to make our mail system work,” Cummings said.

Union officials including Fredric Rolando, president of the National Association of Letter Carriers, say most proposed cuts aren’t necessary because the Postal Service’s financial woes stem from its legal requirement to prepay costs of future retirees’ health benefits. The service has defaulted on those payments for the past two years, $11.1 billion in all, and last week said it won’t be able to afford this year’s payment.

Rolando said his union contributes to candidates regardless from both parties who support the Postal Service.

“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 emailed 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 grass-roots tool,” she said.

The postal unions also are 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.