The most powerful Democratic think tank in Washington, D.C. is raising the stakes of the spending fight that awaits Congress when it returns from Thanksgiving break.

In a memo sent to Capitol Hill on Friday, the Center for American Progress urged Democratic lawmakers to oppose any measure to fund the government unless it includes a provision to protect special counsel Robert Mueller’s investigation into Russian election interference.

“Members of Congress should not vote for any spending bill that does not include provisions protecting the Mueller investigation,” the memo reads. “The DOJ cannot effectively operate with an acting attorney general installed to subvert a lawful investigation, and so Congress cannot fund the DOJ without express provisions to protect against such possible obstruction of justice.”

The memo doesn’t explicitly call for Democrats to shut down portions of the government when several agencies run out of funding on December 7, including the Departments of Homeland Security, Agriculture, and Commerce. Republicans will continue to control both chambers of Congress through the end of the year and can largely determine what funding bill is considered on the floor.

But in drawing a line in the sand over a Mueller protection provision, CAP does increase the chances that a partial shutdown may very well occur. GOP leadership has traditionally relied on Democratic lawmakers to provided the votes needed for government funding legislation to pass. And, already, Republican lawmakers are grappling with President Donald Trump’s push for any measure to include funding for a border wall.

CAP’s argument is that Congress simply can’t punt on the Mueller matter any further. The memo was triggered by the appointment of acting Attorney General Matt Whitaker, who has long expressed hostility to the special counsel probe and even talked about starving it of funding. Whitaker has privately given assurances that he won’t meddle with Mueller’s working. But, in an unprompted answer, Trump brought up his desire to see the probe ended in rationalizing why he elevated Whitaker (who was formerly chief of staff at the Department of Justice) to the role of chief law enforcement officer in the nation.

“The Whitaker appointment broke the DoJ,” said Jesse Lee, a senior adviser at CAP, “we’re demanding it be fixed.”

Specifically, CAP is calling for Congress to pass the “Special Counsel Independence and Integrity Act” as part of the upcoming government funding legislation. That bill has made it through the Senate Judiciary Committee and has bipartisan support and Sen. Jeff Flake (R-AZ) has said he will withhold his vote on Trump’s judicial nominees unless the Senate passes it as a whole. But Majority Leader Mitch McConnell (R-KY) has refused to give it broader consideration, arguing that there is simply no need to protect Mueller at this current moment.

CAP’s demand doesn’t end there. The group is also calling for legislative provisions that would keep Congress abreast of any attempts by Whitaker or other political appointees to interfere in the Special Counsel’s investigative or prosecutorial operations.

“Including these provisions will make it substantially harder for political appointees to interfere in the investigation and also provide Congress with the necessary information to hold oversight hearings and take other action as appropriate,” the memo reads.