Senate Democrats who have long-fought Republican attempts to shut down the government over budget disputes are now threatening to do exactly that.

Democrats said they will block any attempts to include President Trump’s proposed border wall – or other far-right agenda items – in the $1 trillion spending package expected to be announced this week. And, they said, they are not opposed to shutting down the government if Republicans insist on including it.

The warning letter, written by Senate Minority Leader Chuck Schumer and other top Senate Democrats, was addressed to Majority Leader Mitch McConnell. It warned against funding the U.S.-Mexico border wall and other “poison pills” that could be hidden in the budget plan—such as defunding Planned Parenthood, rolling back environmental regulations and allocating additional funding to the Pentagon.

“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 letter read, addressing the bipartisan budget agreement reached in 2015 that set spending levels for defense and non-defense programs.

The letter said including any “poison pill riders” in the bill will have severe consequences.

“We hope our Republican colleagues will work with us in a bipartisan way so that the appropriations process meets the needs of the American people,” Schumer said. “If Republicans insist on inserting poison pill riders such as defunding Planned Parenthood, building a border wall, or starting a deportation force, they will be shutting down the government and delivering a severe blow to our economy.”

McConnell’s office did not immediately issue a statement in response to Schumer’s letter.

But the threat to shut down the government is not one that Republicans are unfamiliar with. Under the Obama administration, Congressional Republicans had major confrontations over budget disputes that ended in a government shutdown or partial shutdown.

A funding gap on October 1, 2013, the first day of the 2014 fiscal year, occurred after disagreements over the Affordable Care Act, or Obamacare, causing the first government shutdown in over 17 years. The shutdown, which lasted 16 days, resulted in the furlough of hundreds of thousands of federal employees.

The Democrats’ letter outlined concerns with funding what they called the “$25 million per mile” border wall, which, they say, has not been resolved by Congress or the Department of Homeland Security.

“There is real concern that the Administration, put simply, has no plan to build the border wall,” the letter read, noting the lack of details the White House has provided in terms of design, eminent domain, location and construction.

The letter said that the Democrats were “committed” to working with the Republicans as they wait for the administration to roll out its first budget plan.

“Rather than pursuing this partisan path,” Schumer said on Monday, “we hope both sides can work together to ensure the government remains funded going forward.”

The Associated Press contributed to this report.