Senate Minority Leader Charles E. Schumer (D-N.Y.) and other Democrats say they won’t accept any attempt to fund the proposed border wall in a spending bill that’s necessary to keep the government open. (Andrew Harnik/AP)

Senate Democrats are preparing to fight back against GOP plans to fund President Trump’s wall on the U.S.-Mexico border, even if it means threatening a government shutdown at the end of April.

Senate Minority Leader Charles E. Schumer (D-N.Y.) and other top Senate Democrats warned GOP leaders Monday that they will block any attempt to use the budget process to begin construction of the wall — especially because the president has not specified how he plans to pay for it.

On Monday, Democrats wrote to Senate GOP leaders that they won’t accept any attempt to include funding for Trump’s proposed border wall in a spending bill necessary to keep the government open past April 28, according to a letter obtained by The Washington Post.

“Given these and other concerns, 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 Democrats wrote.

The formal and carefully written letter to Senate Majority Leader Mitch McConnell (R-Ky.) and Senate Appropriations Committee Chairman Thad Cochran (R-Miss.) is the first clear warning that Democrats are willing to risk a spending showdown to fight back against Trump’s policies. Democrats could easily deny McConnell the 60 votes necessary to pass a spending bill in the Senate. Republicans control 52 seats and will need votes from at least eight Democrats to approve a must-pass bill.

The letter spells out Democrats’ rationale for a shutdown and their hope that Republicans will take the blame if the fight really goes that far. They begin citing estimates that the wall “could cost as much as $25 million per mile” and raise issues that have not yet been hashed out by Congress and the Department of Homeland Security. They lay out several reasons as to why the wall should not be funded in the upcoming spending bill.

“First, many experts believe that such a border wall will not work,” the Democrats wrote. “Second, there is real concern that the Administration, put simply, has no plan to build the border wall.”

They go on to say they are worried that the administration has not provided information about how they will use eminent domain to assert federal control over private land, where the wall will be, how it will be built or if Trump plans to follow through on his promises to make Mexico pay for the wall.

“Finally, there are objections to the construction of a wall from Democratic and Republican members of Congress that represent border states,” they letter said.

Democrats wrote they are ready to start negotiations on a spending bill to keep the government open past the April deadline. But they expect such a deal will still respect a two-year bipartisan budget agreement to equally fund military and domestic programs. That agreement was reached in 2015 by congressional leaders and President Obama and is set to expire in September.

Trump has promised that Mexico will pay for the wall, which was at the center of his presidential campaign. But one top GOP leader isn’t buying that promise: Last week, McConnell answered the question of whether Mexico will pay for the wall with a succinct, “Uh, no.”