Senate Democrats are warning Republicans of a shutdown showdown if President Donald Trump insists on including funding for a wall along the Mexican border in April’s government funding bill.

“We believe it would be inappropriate to insist on the inclusion of such funding in a must-pass appropriations bill that is needed for the Republican majority in control of the Congress to avert a government shutdown so early in President Trump’s Administration,” the Senate Democratic leadership team, along with Appropriations ranking Democrat Patrick J. Leahy of Vermont, wrote in a letter to be circulated Monday.

The letter was directed to Senate Majority Leader Mitch McConnell, R-Ky., and Appropriations Chairman Thad Cochran, R-Miss., but the clear audience was the Trump administration and the new president himself.

An early copy of the letter was obtained by CQ Roll Call.

Led by Minority Leader Charles E. Schumer of New York, the Democrats outline familiar concerns about the wall, which was a key campaign promise of the president. That includes the argument that the wall itself may not deter illegal immigration from Mexico.

They say there has been no plan for construction or for how to get Mexico to fund the wall, as Trump had insisted throughout the campaign.

Aside from that obvious flash point, the Democratic senators are continuing to hold the position that they had under President Barack Obama that any increases in spending above existing caps for non-emergency spending should be equally divided between defense and non-defense accounts.

Senate appropriators in both parties have already grown wary of the prospects that discretionary funding past the end of April for most or all of the budget could be completed through a continuing resolution rather than with an omnibus spending package that reflects the work of members of the Appropriations Committee.

Democrats say they want all of the 12 remaining spending bills to be completed, but even with their reduced leverage with Trump having replaced Obama in the White House, they still indicate opposition to what they consider to be noxious policy riders.

“They should not include poison pill riders such as those that roll back protections for our veterans, environment, consumers, and workers and prohibit funds for critical healthcare services for women through Planned Parenthood,” the Democrats wrote. “We strongly oppose the inclusion of such riders in any of the must-pass appropriations bills that fund the government.”