The Senate is poised to reject Trump's national emergency declaration, but the White House has promised to veto

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President Donald Trump.

• Enough Republican senators have announced they would support a resolution to terminate President Trump's national emergency declaration to build a wall along the US-Mexico border.
• The House already passed the resolution with unanimous Democratic support, as well as 13 Republicans.
• The White House announced they will veto any attempts to terminate the emergency declaration.

WASHINGTON - The Senate is poised to pass a resolution terminating President Donald Trump's use of emergency powers to reprogram federal funds to build additional physical barriers along the United States border with Mexico, after a handful of Republicans announced they would back the Democrat-led legislation.

Trump would ultimately veto the resolution as it lacks a strong enough majority. Still, the resolution coming from a GOP-led Senate would represent a significant blow to the president on one of the most controversial decisions of his administration.

Read more: The lawsuits challenging Trump's national emergency declaration to build the border wall keep piling up

The resolution, first introduced by Democratic Rep. Joaquin Castro of Texas, already passed the House in February. The House resolution passed with unanimous support from Democrats, while 13 Republicans crossed the aisle to rebuke Trump's executive action.

While Republicans were at first critical of using emergency powers to build a border wall, most have backed Trump's decision, including Senate Majority Leader Mitch McConnell. The Senate is now fairly split, with every Democrat backing an identical version of the resolution.

But a few Senate Republicans have left the pack. Sens. Thom Tillis of North Carolina, Susan Collins of Maine, Lisa Murkowski of Alaska have all said they would support a resolution terminating the emergency declaration.

And on Sunday, Trump ally Sen. Rand Paul of Kentucky outlined his support for the resolution in an op-ed for Foxnews.com. Paul's support would tip the scale to give the resolution a simple majority of 51 votes. It is still unclear when a vote will take place.

"Every single Republican I know decried President Obama's use of executive power to legislate. We were right then," he wrote. "But the only way to be an honest officeholder is to stand up for the same principles no matter who is in power."

Paul also noted that he believes Trump's own judicial picks are likely to rebuke him as the issue moves its way through the courts.

"Regardless, I must vote how my principles dictate," Paul wrote. "My oath is to the Constitution, not to any man or political party. I stand with the president often, and I do so with a loud voice. Today, I think he's wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits."

"I understand his frustration. Dealing with Congress can be pretty difficult sometimes," Paul added. "But Congress appropriates money, and his only constitutional recourse, if he does not like the amount they appropriate, is to veto the bill."

Trump will veto the resolution

The White House has already vowed to veto the resolution if it makes it to Trump's desk, noting in a statement of administration policy that it would "undermine the Administration's ability to respond effectively to the ongoing crisis at the Southern Border."

A veto override is still a possibility, but remains a longshot as it requires a two-thirds majority in both chambers of Congress. Veto overrides are rare and difficult to accomplish. For context, Congress did not override a veto from Obama until his final year in office.

Many Republicans who voiced concerns about emergency powers are also faced with the prospect of upcoming re-election fights in 2020.

Sen. John Cornyn of Texas said he will not support the resolution, but believes it will be dealt with as various lawsuits advance through the courts.

"This is obviously no one's first choice," he said.

Meanwhile, Rep. Castro is reportedly mulling a challenge to Cornyn in Texas, setting up what could become one of the more high-profile Senate races in 2020.