Democrats in the U.S. House filed a resolution on Friday seeking to block President Donald Trump’s emergency declaration to build a wall on the southern U.S. border.

The Democratic-led House will consider a resolution to disapprove of the emergency declaration quickly, Speaker Nancy Pelosi has said, and it will be sent over to the Senate as a privileged resolution, which means it will have to get a vote on the Senate floor within 18 days of House passage.

The resolution is the opening salvo in the next phase of conflict between a Republican president and Democrats who capitalized on Trump’s unpopularity to take back the House last November. While Democrats have expressed the desire to get started on their policy agenda, they have also been eager to act as a check on Trump.

Trump last week signed an emergency declaration to divert certain military funding for wall construction, after Congress approved only $1.375 billion of the $5.7 billion he sought in a bipartisan budget bill. The president plans to unilaterally shift nearly $7 billion in federal funds to construct physical barriers along the U.S.-Mexico border.

The resolution should easily pass the House, and could pick up enough Republican votes to pass in the Senate. Several Republican Senators, including Susan Collins of Maine, Marco Rubio of Florida and Rand Paul of Kentucky have expressed doubt about the emergency declaration, and the measure could get enough GOP votes to pass the Senate.

Trump has, however, said he’d veto the measure. Overriding the veto requires a two-thirds majority in the House and Senate. That will be a difficult threshold to overcome given the reluctance of most congressional Republicans to defy Trump, who maintains solid support among GOP voters.

The resolution presents a potential political headache for Senate Majority Leader Mitch McConnell, a Kentucky Republican, who previously said the hoped Trump would avoid using emergency powers to get more money for a wall.

But last week, as Congress faced a deadline to pass a spending bill to avoid another government shutdown, McConnell said on the Senate floor that he told the president he will support the emergency declaration.

In addition to the resolution from the House, 16 states have filed a lawsuit challenging the declaration. The House could also pursue its own legal challenge.