The House will vote Tuesday on whether to to block President Donald Trump’s declaration of a national emergency on the U.S.-Mexico border, forcing Republicans to decide how far they’re willing to go to let him build his long-sought wall.

The resolution is destined to pass the House, where the Democrats hold a majority, and probably will clear the GOP-controlled Senate in coming weeks. But it’s unlikely to get enough support in either chamber to override a promised veto by Trump.

With the votes, both parties will be staking out ground on what’s likely to be one of the more divisive issues in the 2020 election, while also highlighting a power struggle that’s been brewing for years between Congress and the White House over how much control a gridlocked legislature should cede to the executive branch.

It also sets the tone for the next two years of divided government, putting congressional Republicans in the uncomfortable position of opposing their president or defending unilateral White House action they sharply criticized in the Obama administration.

Republican Senator Lisa Murkowski of Alaska said Tuesday she will support the resolution, putting the Democrats within one GOP vote of the number needed to pass the resolution in that chamber if all Democrats and independents vote yes.

Murkowski said that with the emergency declaration, Trump “is overstepping into the legislative prerogative. We as legislators need to be concerned.”

Trump declared a national emergency in a Rose Garden speech on Feb. 15, after Congress averted a second government shutdown by agreeing to spend only $1.4 billion of the $5.7 billion Trump said he needed to build the wall. The emergency allows Trump to divert funds from Defense Department accounts without congressional approval to build the wall.

House Speaker Nancy Pelosi has cast the plan to undo the emergency declaration as her constitutional duty to defend Congress’s authority over taxpayer money.

Constitutional Question

“This isn’t about the border, this is about the Constitution of the United States,” the California Democrat said Monday.“It isn’t about politics, it isn’t about partisanship. This is about patriotism.”

House Republican leader Kevin McCarthy of California, said he agreed with Trump that there is a crisis at the border and that the emergency declaration was a proper exercise of presidential power.

“This is not a wall from sea to shining sea,” said McCarthy, of California. “This is looking at trying to fund 200 miles of barrier that the experts have asked for to protect the border to be able to do their job. He has the right and power to do that.”

While Senate Majority Leader Mitch McConnell and most Senate Republicans appear ready to side with the president, McConnell — when asked Tuesday whether he believes Trump’s emergency declaration is legal — said he didn’t know.

“We’re in the process of weighing that,” he told reporters, adding that Senate Republicans debated the matter at a closed-door meeting with a Justice Department expert and Vice President Mike Pence. McConnell said he “hasn’t reached a total conclusion” about the legality of the declaration.

The majority leader had made clear weeks ago that he hoped to avoid an emergency declaration, but McConnell announced he backed the maneuver on Feb. 14 when Trump agreed to sign a spending bill to prevent a new government shutdown.

Military Construction

The president is using the emergency declaration to use $3.5 billion from military construction accounts. In addition, Trump is attempting to tap $600 million from the Treasury asset forfeiture fund, and $2.5 billion from a Defense Department anti-drug program — moves that don’t rely on the emergency order.

Trump has repeatedly claimed there is a crisis at the U.S.-Mexico border driven by drugs and illegal immigration even though government statistics show apprehensions at the border have been declining for almost two decades and most drug seizures are made at other ports of entry. Democrats accuse Trump of concocting an emergency so he can tell supporters he followed through on his central campaign promise to build the wall..

Veto Override

Democrats and the White House will be closely watching how many House Republicans vote to cancel the emergency declaration. Democrats would need about 55 Republicans to join them in the House to win a veto-proof majority.

“When you see the vote today, there will be nowhere near enough votes to override a veto,” the No. 2 House Republican Steve Scalise of Louisiana said Tuesday.

In the Senate, if all 47 Democrats back the measure, support from just four GOP senators would send it to Trump’s desk, which some lawmakers think could happen. But to override a veto, all Democrats would need to be joined by 20 Republicans. Thirteen GOP senators have signaled they don’t support Trump’s move, but many others say they aren’t inclined to undo it.

Senate Democratic leader Chuck Schumer of New York called Trump’s emergency declaration a “defacement of our constitutional balance of powers for what it seems is largely political purposes.”

Immigration Issue

The call by Pelosi and Schumer for a bipartisan rejection of Trump’s border wall effort is a tough sell for many Republicans who represent states and districts where cracking down on illegal immigration is politically popular. The few Republicans like Michigan Representative Justin Amash who have spoken against Trump’s emergency declaration largely oppose executive overreach — not the border wall itself.

Citing a near 40-year low for illegal border crossings, former national security officials published a statement Monday denying that there is an emergency that “entitles the president to tap into funds appropriated for other purposes to build a wall at the southern border.”

Some Republican former members of Congress also published a letter Monday urging current GOP lawmakers to put the Constitution over their party loyalty by supporting the resolution.

House GOP leaders are instructing members to oppose the resolution, according an aide who spoke on the condition of anonymity.

Maine Republican Senator Susan Collins told reporters last week she would vote for the resolution as long as no objectionable amendments are attached. North Carolina Republican Senator Thom Tillis wrote in a Washington Post opinion piece that he too would vote for the measure, saying, “I cannot justify providing the executive with more ways to bypass Congress.”

Senator Lamar Alexander, a Tennessee Republican, said the declaration is “inconsistent” with the Constitution, while Pennsylvania’s Pat Toomey said it was an attempt to “circumvent” Congress.

The Senate will have 18 calendar days to act on the House resolution or to vote on its own version of the measure. The Senate could amend the House resolution, triggering a conference committee between the two chambers.