House Speaker Nancy Pelosi urged Democrats to back a resolution sponsored by Rep. Joaquin Castro (D-Texas) to terminate President Trump's emergency declaration. Mr. Trump issued his national emergency proclamation last week to access billions of dollars to fund his long-promised wall along the U.S.-Mexico border.

The declaration, Pelosi wrote in a letter to her House colleagues, "undermines the separation of powers and Congress's power of the purse. This power, she wrote, is reserved by the Constitution to the legislative branch, "a branch co-equal to the Executive."

The terminating resolution will be introduced Friday. Pelosi in the letter vowed to "move swiftly to pass this bill." She said it would go through committee within 15 days and considered on the House floor within three days of that.

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"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," Pelosi wrote, adding that members have a responsibility to defend the nation's system of checks and balances "against the President's assault."

Since the president decided to circumvent Congress and act unilaterally to secure more border security funding than the amount approved by lawmakers, congressional Democrats have been signing onto Castro's bill to block Mr. Trump's declaration. On Monday, Castro said his joint resolution had garnered 78 co-sponsors.

"Since the beginning of his term, President Trump has used national security as a pretext to fulfill ineffective campaign promises and inject fear into the American public," Castro said in a statement to CBS News. "This unfounded declaration would take money away from actual, identified national security needs."

The resolution is likely to pass in the Democratic-controlled House, but Democrats face an uphill battle in mustering the necessary votes — let alone a veto-proof majority — to secure its passage in the Senate. Still, the proposal could become a thorny issue for some of the moderate Senate Republicans who have criticized Mr. Trump for bypassing Congress and its constitutional power of the purse.

Mr. Trump's proclamation is already being challenged by a multi-state lawsuit led by California's attorney general and staunch critic of the president, Xavier Becerra.

The White House's proclamation has not only been denounced by Democrats. Some Republicans have criticized the move, including Maine Sen. Susan Collins, who accused Mr. Trump of "usurping congressional authority." Despite voicing concerns about the proclamation before Mr. Trump issued it, most rank-and-file Republicans and the House and Senate GOP leadership have expressed their support since Friday's announcement.

By declaring a national emergency, the White House says it can use $3.6 billion in military construction funds for the construction of a wall along the southwestern border. Through a separate executive order signed last week, the White House will also be able to divert $2.5 billion from counternarcotics initiatives and $601 million from a Treasury Department forfeiture fund.

Rebecca Kaplan contributed to this report.