The challenge for Democrats starts when the proposal goes to the GOP-held Senate. Several Republicans have showed concerns about the precedent set by Trump's declaration. The resolution will put pressure on them to choose between acting on professed worries about expanded executive power, or backing a president with strong support among GOP voters.
At least one GOP senator — Susan Collins of Maine — will vote for a resolution to block the emergency declaration. To reach the majority needed for the measure to clear the Senate, the 47 Democrats in the chamber would then need only three more Republicans to join them. That could easily happen.
However, Trump pledged Friday to veto the legislation if it comes to his desk. The measure would then need veto-proof, two-thirds majorities of 290 and 67 votes in the House and Senate, respectively. Garnering that much support would prove much tougher, considering both House Minority Leader Kevin McCarthy, R-Calif., and Senate Majority Leader Mitch McConnell, R-Ky., have pledged to support Trump's declaration.
"Will I veto it? 100 percent. ... And I don't think it survives the veto," Trump told reporters on Friday afternoon. "We have too many smart people that want border security."
The resolution extends the political and legal dispute over Trump's campaign pledge to build a wall on the U.S.-Mexico border. In several recent lawsuits, including one filed by 16 U.S. states, groups have challenged the emergency declaration. Critics have questioned not only the president's authority to take the step, but also whether migration over the border really constitutes a national emergency.
"All Members take an oath of office to support and defend the Constitution," Pelosi wrote to colleagues this week. "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."
Opponents of the emergency declaration have partly pointed to Trump's comments last week, when he said he "didn't need to" take the action but would rather get money for the wall "much faster."
Trump last week signed a spending bill into law to keep the government running through Sept. 30 and allocate $1.375 billion for building border barriers. Trump had pushed for $5.7 billion for a wall — a demand that led to a record 35-day partial government shutdown in December and January.
Stymied by Congress, Trump aims to use executive authority to construct the wall, using $8 billion total toward the barriers. The sum includes the funds appropriated by Congress, along with money he plans to divert from other departments through executive action.