WASHINGTON — Top Senate Democrats are warning Republicans controlling Congress against adding billions of dollars for President Donald Trump’s U.S.-Mexico border wall to an upcoming $1 trillion-plus catchall spending package.

The warning from Senate Minority Leader Chuck Schumer and others came in a Monday letter to Majority Leader Mitch McConnell of Kentucky. The letter also warns against adding other “poison pills” such as provisions to roll back environmental or consumer protections and urges additional money for domestic programs to match the administration’s planned Pentagon increases.

“We believe it would be inappropriate to insist on the inclusion of (wall) 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,” said the letter, which was provided to The Associated Press. Trump crisscrossed the country last year campaigning for the wall, claiming that he could convince Mexico to pay for it.

READ MORE: Paul Ryan: Congress will pay billions for border wall

At issue is a huge package of leftover spending bills for the fiscal year that began back in October. Congress faces an April 28 deadline to complete the measure and avert a partial government shutdown. It’s separate from Trump’s upcoming partial budget submission for the 2018 budget year that begins on Oct. 1. That proposal is expected on Thursday.

The funding issue is sure to prove difficult to solve — a partial shutdown of the government late next month appears to be a real possibility — and would require a capacity for bipartisan compromise that hasn’t been on display yet in the Trump era.

The letter from Democrats implicitly threatens a filibuster showdown if Republicans try to attach to the must-do legislation controversial Trump agenda items. Any filibuster and shutdown confrontation would spark a high-stakes political battle.

While the letter says it would be “inappropriate” to include money for the border wall, it says Democrats would “strongly oppose” other provisions, including moves against Wall Street regulations or even an attempt to “defund” Planned Parenthood. Schumer demurred when asked last week whether he would lead a filibuster over funding for the wall, but Monday’s letter appears intended to show that Democrats are unified against the idea. Trump also is preparing a request for additional border control and immigration agents.

READ MORE: Here’s what the Mexico border wall looks like now

“All 12 appropriations bills should be completed and 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 health care services for women through Planned Parenthood. We strongly oppose the inclusion of such riders in any of the must-pass appropriations bills that fund the government,” the letter states.

Democrats’ votes are needed to pass the measure through the Senate, unlike the chamber’s agenda so far this year, including Trump’s Cabinet nominees, a looming measure to “repeal and replace” the Affordable Care Act, and a spate of bills to repeal recent Obama administration regulations. But talks have barely begun, and the undermanned Trump administration has yet to weigh in with its expected request for money for the wall and Pentagon buildup.

Money for Trump’s border wall is just the beginning of the political complications facing the must-do measure, which would advance as Republicans and Trump are grappling with their controversial health care law repeal. It could be difficult to avert a clash between Trump and Schumer, and tea party GOP forces are sure to be upset with whatever outcome Democrats eventually agree to.