WASHINGTON - President Donald Trump faces a rare defeat Thursday in the GOP-controlled Senate when lawmakers will decide whether to uphold his controversial national emergency to build his proposed border wall, or knock it down.
The prognosis doesn't look great for Trump.
Trump declared the emergency in February to free up more than $6 billion for the wall - most of it from the Pentagon. Five Senate Republicans have already said they will side with Democrats to support a resolution to rescind it, forcing Trump to sign his first veto.
The president, in a Thursday morning tweet, threatened to veto the resolution should it pass.
"A big National Emergency vote today by The United States Senate on Border Security & the Wall (which is already under major construction). I am prepared to veto, if necessary. The Southern Border is a National Security and Humanitarian Nightmare, but it can be easily fixed!," he said.
More: Who are the Republican senators planning to vote against Trump's border emergency?
White House officials, including Vice President Mike Pence, have scrambled behind the scenes to limit defections. A last-ditch effort by Sen. Mike Lee, R-Utah, to ease some GOP concerns about the emergency collapsed Wednesday after the White House rejected it.
Here's a look at how to read Thursday's vote in the Senate in terms of how many Republicans buck Trump, and what the final count could mean for the president:
0-3 defectors: Victory for Trump
If the White House flips even one of the Republicans who have announced their opposition, it would represent a significant achievement. It would also signal that congressional Republicans see their reelection as being tied to the border wall as much as Trump does.
The GOP lawmakers who have signaled their opposition to the emergency are Sens. Thom Tillis of North Carolina, Susan Collins of Maine, Lisa Murkowski of Alaska, Rand Paul of Kentucky and, as of Wednesday, Lee. Critics say they agree with Trump that the border needs more resources but they say Trump's emergency declaration steps on the nation's founding principle of the separation of powers.
"My concern is really about the institution of the Congress," Murkowski said recently. "The power of the purse rests with the Congress."
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."
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Contributing: Deborah Berry
This article originally appeared on USA TODAY: Mild rebuke or open revolt? Border wall emergency vote a test of Trump's strength