House Speaker Nancy Pelosi (D-Calif.) said Wednesday she would introduce legislation later this week to block President Donald Trump’s national emergency declaration to build his wall along the southern border, moving “swiftly” to place it on the president’s desk.

In a letter to fellow lawmakers, Pelosi urged colleagues to co-sponsor the resolution written by Rep. Joaquin Castro (D-Texas). Under the National Emergencies Act, which Trump is citing to claim authority to declare a border emergency, Congress can vote to disapprove and essentially block the declaration. The resolution is likely to pass the House and possibly the Senate, but Trump could still veto it.

It’s unlikely that there are enough supporters in Congress to override his veto.

“President Trump’s emergency declaration proclamation undermines the separation of powers and Congress’s power of the purse, a power exclusively reserved by the text of the Constitution to the first branch of government, the Legislative branch, a branch co-equal to the Executive,” Pelosi wrote in a “Dear Colleague” letter Wednesday.

Democratic leaders had planned to move on the resolution earlier this week, but Pelosi’s effort to push it through as quickly as possible is notable.

Trump declared the national emergency last week in an effort to unilaterally seize funding for his long-promised wall. Attempts to negotiate with Congress to obtain $5.7 billion for the barrier failed, resulting in the nation’s longest-ever government shutdown. Congress eventually passed a spending bill that included $1.375 billion for border barriers, which Trump signed but then said he would use his executive powers to more swiftly fund his wall project.

“I could do the wall over a longer period of time ― I didn’t need to do this,” the president said last week. “But I’d rather do it much faster.”

The move all but assured a prolonged court battle, and a coalition of 16 states filed a federal lawsuit on Monday arguing that Trump’s use of a national emergency was unconstitutional.

“Contrary to the will of Congress, the President has used the pretext of a manufactured ‘crisis’ of unlawful immigration to declare a national emergency and redirect federal dollars appropriated for drug interdiction, military construction and law enforcement initiatives toward building a wall on the United States-Mexico border,” the suit, led by California Attorney General Xavier Becerra, read.

Pelosi on Wednesday argued that members of Congress had a duty to support and defend the laws of the United States but said Trump had gone “outside the bounds of law to try and get what he failed to achieve.”

“We have a solemn responsibility to uphold the Constitution, and defend our system of checks and balances against the President’s assault,” she concluded.

Lawmakers have until 3 p.m. EST Thursday to sign on to Castro’s resolution. It’s unclear how many Republicans will support it, and The Washington Post said Trump would veto the resolution.

Politico notes that 100 House Democrats have already signed on.