With the GOP-controlled Senate set to vote today on whether to strike down Donald Trump’s border wall emergency declaration, it’s already guaranteed the president will face a rare defeat from within his own party.

And the prognosis is looking worse and worse as Sen. Mitt Romney, R-Utah, came forward as the fifth Republican poised to vote down Trump’s emergency declaration some time today after lunch.

Romney joins Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Rand Paul of Kentucky and Thom Tillis of North Carolina, who have all said they’ll vote with the Democrats. A simple majority of 51 is enough to pass the resolution blocking the president’s emergency declaration, and there are now at least 52 votes against it.

“I will vote today for the resolution of disapproval,” Romney said in a statement. “This is a vote for the Constitution and for the balance of powers that is at its core. For the Executive Branch to override a law passed by Congress would make it the ultimate power rather than a balancing power. This is not a vote against border security. In fact, I agree that a physical barrier is urgently needed to help ease the humanitarian crisis at the southern border, and the administration already has $4.5 billion available within existing authority to fund a barrier — even without an emergency declaration.

“I am seriously concerned that overreach by the Executive Branch is an invitation to further expansion and abuse by future presidents. We experienced a similar erosion of congressional authority with President Obama’s unilateral immigration orders — which I strenuously opposed. In the case before us now, where Congress has enacted specific policy, to consent to an emergency declaration would be both inconsistent with my beliefs and contrary to my oath to defend the Constitution.”

The question for the president is how many more will join them.

Trump can veto the resolution, which easily passed the Democrat-controlled House, requiring a two-thirds super majority in each chamber of Congress to override his veto. How many GOP votes against his declaration today could signal whether or not his veto will stand.

Trump’s veto will likely withstand being overturned, and will likely end up for the Supreme Court to decide its legality.

Trump implored Republicans again Thursday morning to stand with him.

The White House is reportedly bracing for as many as 14 Republicans to join the Democrats, which would be a disastrous and embarrassing defeat for the president.

Per USA Today:

4-7 defectors: Rebuke

Democrats need four Republican votes to rescind Trump’s emergency – and they currently have those votes. Because the Democratic-led House has already approved the resolution, a Senate vote could send the measure to the president’s desk.

Under the National Emergencies Act, Congress can rescind a presidential emergency with a simple majority. But Trump can veto the resolution, and the White House has already said he will in this case. It’s unlikely opponents of the emergency would be able to garner enough support to overturn a veto.

Roughly a dozen Senate Republicans remain on the fence, adding to the drama. Some, such as Lamar Alexander of Tennessee, have pushed Trump to take another course. Lee had proposed a compromise to tighten the rules around future emergencies.

But the concept struggled to gain support from the White House or Republicans opposed to Trump’s emergency.

Collins told reporters that the Lee proposal did “not address the current problem that we have, where the president, in my judgment, is usurping congressional authority.”

A White House spokesman did not respond to questions about the Lee proposal.

“It is inconsistent with the U.S. Constitution because, after the American Revolution against a king, our founders chose not to create a chief executive with the power to tax the people and spend their money any way he chooses,” Alexander said recently.

More than 8 defectors: GOP revolt

The closer the tally of Republican defectors gets to double digits, the bigger problem Trump could have in explaining his expected veto.

Trump insists he is using the emergency powers available to him under the law, and correctly notes that past presidents have signed dozens of emergency declarations. But critics say Trump’s emergency declaration, by contrast, is a response to his inability to convince Congress of the need for billions in wall funding.

And that is unusual. The emergency declaration is already the subject of several lawsuits.

Trump declared the emergency after Congress included only $1.375 billion for the border wall in their most recent government funding measure. The amount was far short of the $5.7 billion Trump initially insisted on during the 35-day government shutdown that ended in January only after he relented.

Trump’s job approval remains high within the GOP, a factor that could limit defections. But while a majority of Republicans back the emergency declaration, some two-thirds of Americans overall opposed it, according to several polls earlier this year.

Senate Majority Leader Mitch McConnell did not answer when a reporter asked him Wednesday if passage of the resolution would represent a rebuke of the president.

“I think everybody in my conference is in favor of the president’s position on the wall and on border security,” the characteristically understated GOP leader said. “It is no secret that the use of the national emergency law has generated a good deal of discussion.”