Lawmakers are currently working furiously to hash out the details on a border security deal, and one thing that’s caught up in negotiations is a bill that would guarantee back pay for federal contractors affected by the recent government shutdown.

Unlike other government employees who received back pay for the time they were furloughed during the shutdown — which lasted a whopping 35 days — many federal contractors simply have a massive hole in their 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 chalk up this gap in pay as a loss.

A bill introduced by Sen. Tina Smith (D-MN) seeks to change that, and Democrats are pushing to get it attached to a final spending package. This bill would guarantee back pay for federal contractors by calling on agencies to simply pay out funds that they would have spent anyway, had the government not been shut down.

This bill is proving to be a point of contention in negotiations, however, and two Democratic sources say that it’s getting pushback from Republicans. One potential reason is that it would ask agencies to use funds that they could otherwise save.

According to one source familiar with the negotiations, the objection to the bill’s inclusion in a final spending package is coming directly from Senate Majority Leader Mitch McConnell, an assertion that the Republican leader’s office has pushed back on.

His spokesperson, David Popp, disputed this characterization and emphasized that the majority leader’s involvement in taking up legislation was separate from appropriator’s negotiations on the subject. Another congressional aide noted, however, that the additions to the border security deal are being negotiated at a leadership level.

Two Republican senators, Susan Collins (ME) and Lisa Murkowski (AK), had previously signed on to back the bill.

If the legislation is ultimately included in a spending package and approved by Congress, it could have a massive impact on thousands of workers, many of whom struggled with bills on medication, child care, and food for their families during the shutdown.

Up to 580,000 contractors, including cafeteria workers, security guards, developers, and IT consultants, could be missing out on back pay because of the impasse, according to NYU public service professor Paul Light. (Depending on the company they work for, some contractors may have received their pay uninterrupted during the shutdown.)

The text of the spending package — one that’s expected to both keep the government open and cover funding for nine federal departments — is expected later Wednesday afternoon and is set to contain more than $1.3 billion for border fencing. The contractor back pay bill is among a slew of outstanding sticking points.

As lawmakers seek to prevent a future shutdown, they could provide major protections to workers who were hurt by the past one. What remains to be seen is whether they can work out their differences to do so.