More than 30 Democratic senators, led by Sen. Chris Van Hollen (D-MD), want Congress to include back pay for contractors who were affected by the partial government shutdown in an upcoming disaster relief package.

The 800,000 federal government employees who were caught up in the shutdown in December and January got back pay after the 35-day impasse but as many as 580,000 contractors did not. Many of them still have yet to receive any funds to make up for the gaping hole left by their January paychecks.

Because they work for third-party companies that the government pays for their services, contractors don’t get paid when these services aren’t used. During past shutdowns, contractors have been forced to simply chalk up this lack of pay as a loss.

Democratic senators, along with Republican Sen. Susan Collins (ME), have signed on to the letter that pushes appropriations leaders to address this gap.

“The people who were severely harmed by this are still struggling,” Van Hollen told Vox. “You have these employees who went without a paycheck for 35 days. They are still facing extreme financial hardship.”

In a Friday letter to Senate Appropriations Chair Richard Shelby (R-AL) and ranking member Patrick Leahy (D-VT), the senators urge them to consider attaching contractor back pay to supplemental funding measures for the 2019 fiscal year or other spending bills in the works. One of these measures is a disaster relief bill that lawmakers are currently weighing on the Hill. If contractor back pay ends up included as part of this package, it would mark the first time Congress has ever approved it after a shutdown.

But Democratic lawmakers face a tough battle: Both the White House and some Senate Republicans have said they oppose contractor back pay.

Contractor back pay was blocked by the White House earlier this year due to cost concerns. Senators think they have an answer.

During the debate over an omnibus spending package lawmakers passed earlier this year, federal contractor back pay was discussed but not ultimately included. That was largely due to pushback about its cost from the White House and some Senate Republicans.

An informal score from the Congressional Budget Office, which evaluates the costs associated with different pieces of legislation, estimated the costs at $1 billion.

But Democrats argue that the cost is actually lower than that because some contractor pay was previously built into federal agency budgets, Van Hollen says.

Agencies have already budgeted a set amount for contractors, which include janitors, cafeteria workers, developers, and consultants. So the actual back pay for workers would not be an additional cost. Democrats say they’re simply calling on agencies to pay out money in their budgets they would have spent anyway if the shutdown hadn’t happened.

“If you budget to pay security contractors and janitorial services ... those payments are still included in the budget [even if your agency closed down],” Van Hollen says. Plus, agencies already have mechanisms to pay contractors, meaning the administrative costs could also likely be kept to a minimum, he notes.

The push to add contractor back pay to disaster relief funding is also notable because both would be one-time funding expenditures. Disaster relief funding does not interfere with agency spending caps, and contractor back pay could be treated the same way, Van Hollen says.

Some Senate Republicans have told Vox they feel that contractors are private employees, and shouldn’t necessarily receive back pay if they didn’t work during the shutdown, unlike furloughed federal employees.

Van Hollen and other Democrats vigorously disagree.

“These are individuals doing work on behalf of the federal government,” he says. “It is simply not fair to put the entire financial burden of something they couldn’t control on them.”