House Speaker Nancy Pelosi on Wednesday encouraged members to support a joint resolution to try to block President Trump’s recent move to declare a national emergency on the southern border and vowed that the House will move “swiftly” to pass it.

“All Members take an oath of office to support and defend the Constitution,” Mrs. Pelosi wrote in a letter to colleagues. “The President’s decision to go outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process violates the Constitution and must be terminated.”

The office of Rep. Joaquin Castro, Texas Democrat, has been circulating the resolution for signatures. Mrs. Pelosi said it will be formally introduced on Friday, and that the deadline to sign on as an original cosponsor is Thursday at 3 p.m.

Under the National Emergencies Act, which Mr. Trump invoked in order to unlock about $3.6 billion in military construction accounts for a border barrier, the House and Senate can take up and pass a resolution disapproving of an emergency declaration on simple majority votes in both chambers.

That shouldn’t be too big of a lift in the Democratic-controlled House, but at least four GOP senators would have to take a vote on the floor against the president in order for it to pass the Senate, where Republicans hold an effective 53-47 majority.

In her letter, Mrs. Pelosi flatly declared that the resolution “will be referred to the Senate and then sent to the President’s desk” after the House passes it.

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

Even if the resolution passes both chambers, Republicans have been skeptical that there would be enough support to override a presidential veto, which takes a two-thirds majority in the House and the Senate.

The White House has identified a total of about $8 billion to go toward barriers along the U.S.-Mexico border, with the rest coming from a combination of the recently-passed spending package, counter-narcotics money, and treasury forfeiture funds.