A group of conservative House Republicans on Wednesday urged Senate Minority Leader Mitch McConnell (R-Ky.) to effectively force the federal government to run out of money this week unless lawmakers move to defund vaccine mandates imposed by President Joe Biden—fueling uncertainty over whether Congress will be able to avert a government shutdown just three days before funds are set to dry up.

In a letter on Wednesday, the House Freedom Caucus asked McConnell to “use all procedural tools” at his disposal to deny the timely passage of a short-term government funding measure, known as a continuing resolution, unless it prohibits funding “in all respects” for the vaccine mandates Biden introduced this fall for companies with 100 or more employees and healthcare workers, among others.

Though some of the mandates have already been challenged and delayed by courts, the group insisted Senate Republicans should use the looming government shutdown, which would start after funding expires Friday night, as leverage to encourage Democrats to back down on the orders. 

In response to the letter, Senate Majority Leader Chuck Schumer (D-N.Y.) told reporters he’s made “good progress” in talks with McConnell, but said a forced shutdown could spark “total chaos,” adding: “It’s up to the leaders on both sides to make sure that doesn’t happen.”

It’s still unclear how Republican senators, who would need to unanimously advance the continuing resolution before Friday to avoid a shutdown, will respond, but on Tuesday McConnell, whose office did not immediately respond to Forbes‘ request for comment, sought to temper concerns by vowing the government “won’t shut down.”

If Democrats don’t agree to withhold funding to implement the vaccine mandates, it’s possible at least one of the senators necessary for an objection could delay a streamlined funding resolution, forcing a slew of procedural measures that would trigger a shutdown through at least Sunday, one Democratic staffer told Politico. 

Last month, ten Republican Senators signed a letter blasting Biden’s “cruel campaign to punish unvaccinated Americans” and pledging to reject any legislation—including a continuing resolution—that funds or enables enforcement of the mandates.