President Trump on Thursday formally announced that he would pursue a pay freeze for civilian federal employees in 2019, increasing the likelihood of a fight with Congress next month.

Trump needed to formalize an alternative compensation plan for 2019 by tomorrow, or else a 25 percent increase in locality pay and 2.1 percent increase in across-the-board raises would automatically kick in in accordance with the 1990 Federal Employees Pay Comparability Act. He initially floated the prospect of a pay freeze for civilian feds in his fiscal 2019 budget proposal earlier this year.

“I have determined that for 2019, both across-the-board pay increases and locality pay increases will be set at zero,” Trump wrote. “These alternative pay plan decisions will not materially affect our ability to attract and retain a well-qualified federal workforce.”

The announcement comes weeks after the Senate included a 1.9 percent pay increase for federal workers in one of its minibus spending bills, and after the House in its own legislation did not broach the issue of federal compensation, effectively endorsing the pay freeze. Next week, both chambers will begin negotiations to iron out the differences between the two versions of the spending package.

If Congress chooses to adopt the Senate’s proposed pay increase, that would effectively override Trump’s pay freeze. Although the White House has announced its opposition to the pay increase, it has not issued a veto threat over the provision.

In his letter to Congress, Trump said the federal government must move toward a pay for performance model. Although not mentioned in the document, the White House has repeatedly asked Congress for a $1 billion interagency workforce fund to pay for agencies to implement performance-based pay pilot programs, a proposal omitted from both the House and Senate appropriations bills.

“In light of our nation’s fiscal situation, federal employee pay must be performance-based, and aligned strategically toward recruiting, retaining and rewarding high-performing federal employees and those with critical skill sets,” Trump wrote. “Across-the-board pay increases and locality pay increases, in particular, have long-term fixed costs, yet fail to address existing pay disparities or target mission critical recruitment and retention goals.”

In a statement Thursday afternoon, Senate Appropriations Committee Vice Chairman Patrick Leahy, D-Vt., suggested the White House may not have the last word on this:

“The United States Senate has a long history of supporting the federal workforce by providing competitive pay, which attracts and retains talented women and men to the United States Government, who work every day to serve Americans. On August 1, the Senate passed a pay raise of 1.9 percent with strong bipartisan support in a vote of 96 to 2. The Senate will continue to move forward on appropriations bills that have bipartisan support, that are at spending levels agreed to in the bipartisan budget deal, and that reject poison pill riders and controversial authorizing language,” Leahy wrote.

This story was updated with the statement from Sen. Leahy.