WASHINGTON — With tens of thousands of sick nuclear weapons workers thwarted in their bids to win compensation from the U.S. government, Congress is under growing pressure to investigate what went wrong with a $12 billion federal program aimed at helping them.

“It’s disgraceful how the affected workers have been treated by a system that’s overly bureaucratic,” said Colorado Democratic Rep. Jared Polis, one of six members of the House of Representatives now pushing for hearings, along with Democratic Sens. Claire McCaskill of Missouri and Tom Udall of New Mexico.

Udall said the program had been “plagued with delays and bureaucratic hurdles,” forcing too many workers to spend their final years battling both illness and the government.

“Too often, workers die waiting for compensation that they never receive,” he said.

The calls for a congressional probe come after an investigation by McClatchy last year found that fewer than half of those who’ve applied for compensation have received any money, even though the program’s costs have ballooned.

McClatchy reported that 107,394 current and former nuclear workers have sought compensation for cancers and other illnesses after working at 325 current and defunct nuclear sites across the nation.

Overall, the federal government had made payments to more than 53,000 sickened or dead workers under the Energy Employees Occupational Illness Compensation Program, which launched in 2001. In many cases, the money went to surviving relatives who’d taken on the battles of their deceased loved ones.

And 7,762 workers died after filing for compensation, getting nothing, according to McClatchy’s analysis of government records.

“This is clearly a tangled set of issues that Congress should take another look at,” McCaskill said Thursday. “The folks who worked on the front lines of developing and maintaining America’s nuclear capability earned the right to know that if that work made them sick, their government won’t turn its back on them.”

Missouri Democratic Rep. Emanuel Cleaver, who once worked in a nuclear weapons plant in Kansas City, first requested congressional action in December, asking for “a full and unrestrained investigation” in a letter to the House Committee on Government Reform.

His push is now backed by Sens. McCaskill and Udall and five other House members: Democrats Polis, Ed Perlmutter of Colorado, Ben Ray Lujan of New Mexico and James Clyburn of South Carolina, and Republican Rep. Ed Whitfield of Kentucky.

“It is time to do it,” Perlmutter said, adding that Congress has many questions to answer: “Is this working? Is it helping the people we want to help? Is it helping people who don’t deserve it?”

In too many cases, he said, the compensation program, which is run by the U.S. Department of Labor, is “weighted against the worker.”

McClatchy’s investigation also raised questions about worker safety as the U.S. gears up to spend $1 trillion in modernizing its nuclear arsenal over the next 30 years. McClatchy found that stronger safety standards have not stopped accidents or exposure to radiation or other toxins, and reported that contractors for the Department of Energy have paid tens of millions in fines for safety violations related to radiation at nuclear facilities around the country.

“Hearings on the program are long overdue,” said Terrie Barrie, of Craig, Colo., founding member of the Alliance of Nuclear Worker Advocacy Groups. She thinks that federal officials are trying to cut costs, and she said the program had deteriorated since 2007, when Congress held its last hearings on it.

“This compensation program needs a bright light shone upon it. There should be some kind of interest from Congress to say: ‘What’s going on here?’” Barrie said.

Clyburn said he was concerned about “ongoing health and safety problems faced by workers at the Savannah River Site” in South Carolina and that he supported hearings to look into the issue.

Cleaver said the number of employees who’d been compensated at the Kansas City plant, where he’d worked as a young man, seemed “invisibly low.” The approval rate is 23 percent, less than half the national average, according to an analysis of government data McClatchy obtained through the Freedom of Information Act.

Barrie and some members of Congress fear that the Department of Labor will make it even tougher for sick workers exposed to toxic chemicals in the last two decades.

A new department policy puts a higher burden of proof on employees to document their illnesses from any post-1995 exposures. Claims examiners have been ordered to conclude that workers have not had any significant exposure to toxic substances unless they can provide evidence to the contrary.

“There is a higher bar, and I don’t like it,” said Perlmutter, adding that it will make it harder for employees at Colorado’s Rocky Flats site to get help from the federal government if they’re struck by cancer or other illnesses.

Lujan said the compensation program had become “far too difficult to navigate, causing too many workers to be denied benefits.” His father, former New Mexico House Speaker Ben Lujan, blamed the lung cancer that killed him in 2012 on exposure to asbestos while he worked at the Los Alamos National Lab in New Mexico.

“I believe that Congress must do everything possible to conduct vigorous oversight of this program to ensure that nuclear workers are provided the health care and benefits that they have earned,” Lujan said.

Whitfield said that many workers at the Paducah Gaseous Diffusion Plant in Kentucky had faced challenges in “weaving through the federal government maze to claim benefits they deserve.”

He said a newly created advisory board that would be seated this year would help provide better oversight of the compensation program and should expedite payments. But he said he had backed congressional hearings in the past and would do so again.

Polis said he had hoped the advisory board Congress created last year would be up and running by now. But he said: “We’ve been very disappointed with how this process has played out so far.”

Cleaver is following up his request for an investigation with a letter to the Department of Labor. Along with Udall, Polis, Perlmutter, Lujan and Whitfield, he is requesting that the department extend the public comment period, currently scheduled to end Jan. 19, to allow more input before finalizing proposed changes to the compensation program. Critics fear the new rules would make it harder to qualify.

Part of the reason the extension is sought is to give the new advisory board time to review the changes, as well.

“The men and women who worked at these nuclear facilities have sacrificed their health, and even their lives, for their country,” Cleaver said, pledging to work “to make sure that the claims process is timely, transparent and accurate.”

The Department of Labor defended its management of the compensation program, saying it has sped up its processing time for claims and that its acceptance rate for claims has increased since 2008.

“The Department of Labor’s goal is to accept as many claims as possible within the confines of the law,” said spokeswoman Amanda McClure.

And, she said, “we look forward to engaging all interested parties to work on ways to improve the program and help qualified former workers receive the benefits that they deserve.”

McCaskill’s office has asked for a briefing on the program from the Labor Department.