The Trump administration is urging congressional Republicans to deny funding for sanctuary cities in a must-pass spending bill later this month, a stance that would almost certainly increase the threat of a government shutdown.

When Congress returns from a two-week recess, lawmakers will be facing a deadline of April 28 to pass legislation that funds the government through September, and congressional Republicans know that Democratic support will be needed to get a bill passed, particularly in the Senate where Republicans hold a 52-48 majority.

Speaker Paul Ryan (R-WI) has already committed to making sure there aren’t any poison-pill amendments in the spending bill that could raise the threat of a government shutdown. Ryan has already guaranteed that provisions to fund President Trump’s wall at the Mexico border or defund Planned Parenthood won’t be in the short-term spending bill.

But President Trump’s budget director Mick Mulvaney is pressing Ryan and other Republican leaders to include language that restricts federal funding grants for sanctuary cities that refuse to enforce federal immigration policies.

Senate Minority Leader Chuck Schumer (D-NY) said negotiations to fund the government are going well but the sanctuary cities funding language would be a deal-breaker.

“We hope our Republican colleagues won’t insist on things that will cause a government shutdown, but talks are going pretty well right now,” Schumer said on a call with reporters.

Senate Majority Leader Mitch McConnell (R-KY) vowed there won’t be a shutdown. “Most things in the Senate do require 60 votes,” he said. “Democrats are not irrelevant. The first big test of that will be the funding bill when we get back.”