WASHINGTON (AP) — Top Senate Democrats are warning Republicans controlling Congress against adding billions of dollars for President Donald Trump's U.S.-Mexico border wall to a coming $1 trillion-plus spending package.

The warning from Senate Minority Leader Chuck Schumer and others came in a letter Monday 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's proposal for the wall was a centerpiece of his presidential campaign and he claimed he could persuade Mexico to pay for it.

The letter from Democrats implicitly threatens a filibuster showdown — and potential government shutdown — if Republicans try to attach controversial Trump agenda items to the must-do legislation.

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

The funding issue will be difficult to solve — with the potential government shutdown increasing the political pressure — and would require a capacity for bipartisan compromise that hasn't been on display yet in the Trump era.

Schumer voted to authorize the existing border fence a decade ago during the Bush administration and voted for legislation funding its construction. The letter says it's premature to rush Trump's upgrades through.

"The Administration, put simply, has no plan," the letter says.

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 money 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.

"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," the letter says. "We strongly oppose the inclusion of such riders in any of the must-pass appropriations bills that fund the government."

Democrats' votes are needed to pass the measure through the Senate. 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 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 get behind.