House Speaker Nancy Pelosi could move to block President Trump from invoking a national emergency to build a border wall, forcing Senate Republicans to choose between preserving congressional power and crossing the White House.

The California Democrat, under a seldom-used statute, could put a binding “resolution of disapproval” on the House floor to counter Trump should he claim constitutional powers to unilaterally build a border wall. The president is threatening action if Congress refuses his demand for $5.7 billion in border wall funding. Friday, Trump expressed skepticism that a bipartisan commission seeking a deal on the wall to avoid another government shutdown would succeed, saying there was a "good chance" he would declare an emergency.

The lawmakers are desperate to avoid another standoff over immigration and are worried about political anger and even retribution from the pro-Trump base. Yet they are wary of endorsing an unprecedented expansion of presidential emergency powers. Some fear the powers, once in a Democratic president's hands, could be used to justify all sorts of progressive priorities.

The resolution would almost assuredly pass the Democratic House. Then, in a quirk of the law — the Congressional Review Act — Senate Majority Leader Mitch McConnell, R-Ky., would essentially be forced to hold a floor vote, with only a simple majority required for passage rather than the customary 60 votes.

[Related: 'A WALL is a WALL': Trump mocks Congress for talking about fences, barriers]

Trump could veto the measure if it passed. But that would be almost besides the point. Some Republicans are in a political pickle either way.

“We have a lot of folks who are uncomfortable with [an emergency declaration] for different reasons, some of which are constitutional and separation of powers,” Senate Majority Whip John Thune, R-S.D., said Thursday. “We prefer that not be the direction that the administration chooses to go. But the Democrats have to be willing to play ball here if we’re going to get a deal.”

Thune is not ruling out that several Senate Republicans could join with the Democrats to block Trump from declaring an emergency. A second Republican senator, speaking on condition of anonymity, predicted the president would suffer major defections if a vote on a resolution of disapproval is held.

Trump agreed last Friday to reopen the government after a historic 35-day shutdown, caving to Democrats who refused his demand that they provide wall funding before opening the government. This week, bipartisan negotiations began in a special, bicameral congressional committee. They have three weeks before funding runs out again.

Pelosi's office declined to comment.