House Speaker Nancy Pelosi is urging Democrats to sign onto a resolution to block President Trump’s decision to declare a national emergency at the southern border and said the House will vote on the measure next month.

“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, D-Calif., wrote to House Democrats this week.

Trump last week declared a national emergency in order to take $3.6 billion out of the military construction budget and use it to construct physical barriers along the southern border.

Rep. Joaquin Castro, D-Texas, last week introduced a joint resolution to revoke the national emergency.

Judiciary Committee Chairman Jerrold Nadler, D-N.Y., plans to hold a hearing on Trump’s declaration when Congress returns next week.

Pelosi urged lawmakers to sign on to Castro’s measure.

Pelosi said in the letter to Democrats that the House “will move swiftly to pass this bill, reporting it out of committee within 15 calendar days and considering it on the Floor within 3 calendar days following that.” That timeline sets up a vote in March.

[Related: 16 states sue over Trump border national emergency]

The House, where Democrats are in the majority, is very likely to pass the resolution, and that will force the Senate to take up the measure and vote on it within 15 days. A handful of senators have publicly criticized the national emergency declaration, and it would only take four GOP senators to vote with Democrats, pass it, and send the measure to Trump’s desk with a simple majority.

“All Members take an oath of office to support and defend the Constitution,” Pelosi said in the letter. “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.”

Senate Minority Leader Chuck Schumer, D-N.Y., also senators to support the resolution.

“If the president’s emergency declaration prevails, it will fundamentally change the balance of powers in a way our country’s founders never envisioned," he said. "That should be a serious wake up call to senators in both parties who believe in the constitutional responsibility of Congress to limit an overreaching executive."

Trump is likely to veto the resolution, which would send it back to the House. Lawmakers could then attempt to override the veto, but that requires a two-thirds majority and cooperation from 50 Republicans, which appears unlikely.