Sen. Rand Paul (R-KY) speaks to members of the press on health care September 25, 2017 on Capitol Hill in Washington, DC.
Alex Wong/Getty Images
Sen. Rand Paul (R-KY) speaks to members of the press on health care September 25, 2017 on Capitol Hill in Washington, DC.
Now playing
01:08
Senators predict more to vote against national emergency
U.S. Marines conduct an operation to clear a village of Taliban fighters in July 2009 in Mian Poshteh, Afghanistan.
Joe Raedle/Getty Images
U.S. Marines conduct an operation to clear a village of Taliban fighters in July 2009 in Mian Poshteh, Afghanistan.
Now playing
03:19
Biden to announce Afghanistan withdrawal by September 11
roger wicker
CNN
roger wicker
Now playing
04:52
Sen. Wicker on Biden's infrastructure plan: Not ruling out tax hike
Representative Matt Gaetz (R-FL) arrives for a House Armed Services Subcommittee hearing with members of the Fort Hood Independent Review Committee on Capitol Hill on December 9, 2020 in Washington, DC.
Samuel Corum/Getty Images
Representative Matt Gaetz (R-FL) arrives for a House Armed Services Subcommittee hearing with members of the Fort Hood Independent Review Committee on Capitol Hill on December 9, 2020 in Washington, DC.
Now playing
03:02
Sources say Gaetz was denied meeting with Trump
CNN
Now playing
02:58
Avlon: This shows that crazy has a constituency
CNN
Now playing
07:27
CNN anchor pushes back on Texas state lawmaker's defense of voting bill
CNN
Now playing
01:12
Tapper asks Buttigieg for infrastructure plan timeline
Now playing
02:48
GOP governor calls Trump's RNC remarks 'divisive'
WASHINGTON, D.C. - APRIL 19, 2018:  The U.S. Supreme Court Building in Washington, D.C., is the seat of the Supreme Court of the United States and the Judicial Branch of government. (Photo by Robert Alexander/Getty Images)
Robert Alexander/Getty Images
WASHINGTON, D.C. - APRIL 19, 2018: The U.S. Supreme Court Building in Washington, D.C., is the seat of the Supreme Court of the United States and the Judicial Branch of government. (Photo by Robert Alexander/Getty Images)
Now playing
02:39
SCOTUS blocks California Covid restriction on religious activities
rep jim clyburn georgia voting law jim crow sot sotu vpx_00000000.png
rep jim clyburn georgia voting law jim crow sot sotu vpx_00000000.png
Now playing
02:13
Rep. Clyburn blasts GA voting law: It's the 'new Jim Crow'
Joe Manchin
CNN
Joe Manchin
Now playing
02:03
'I never thought in my life ...' Why Manchin won't walk away from bipartisanship
Gaetz speaks to members of the media outside the hearing Michael Cohen, former attorney and fixer for President Donald Trump, testifies at before the House Committee on Oversight and Reform at Rayburn House Office Building February 27, 2019 on Capitol Hill in Washington, DC. Last year Cohen was sentenced to three years in prison and ordered to pay a $50,000 fine for tax evasion, making false statements to a financial institution, unlawful excessive campaign contributions and lying to Congress as part of special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential elections.
Alex Wong/Getty Images
Gaetz speaks to members of the media outside the hearing Michael Cohen, former attorney and fixer for President Donald Trump, testifies at before the House Committee on Oversight and Reform at Rayburn House Office Building February 27, 2019 on Capitol Hill in Washington, DC. Last year Cohen was sentenced to three years in prison and ordered to pay a $50,000 fine for tax evasion, making false statements to a financial institution, unlawful excessive campaign contributions and lying to Congress as part of special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential elections.
Now playing
06:11
'Bombastic, antagonistic, unapologetic': A look at Gaetz's political career
Former House Speaker John Boehner attends a ceremony to unveil a portrait of himself on Capitol Hill, Tuesday, Nov. 19, 2019 in Washington.
Michael A. McCoy/AP
Former House Speaker John Boehner attends a ceremony to unveil a portrait of himself on Capitol Hill, Tuesday, Nov. 19, 2019 in Washington.
Now playing
02:42
Boehner says Republican colleague held 10-inch knife to his throat outside House floor
President Joe Biden, accompanied by Vice President Kamala Harris, and Attorney General Merrick Garland, speaks about gun violence prevention in the Rose Garden at the White House, Thursday, April 8, 2021, in Washington.
Andrew Harnik/AP
President Joe Biden, accompanied by Vice President Kamala Harris, and Attorney General Merrick Garland, speaks about gun violence prevention in the Rose Garden at the White House, Thursday, April 8, 2021, in Washington.
Now playing
02:05
Biden calls for ban on assault weapons
CNN
Now playing
02:22
Biden: High-speed internet is infrastructure
AFP/Getty Images
Now playing
03:24
Donald Trump breaks his silence on Matt Gaetz
(CNN) —  

President Donald Trump is expected to suffer an embarrassing rebuke when the Senate votes to block his national emergency declaration to build a wall on the US southern border, forcing Trump to issue the first veto of his presidency on what he considers to be his top domestic issue.

Ahead of that expected outcome, Republicans, who control the Senate, are wary of piling-on, torn between wanting to protect the President and Congress’ power of the purse.

At least four Republican senators – Rand Paul of Kentucky, Lisa Murkowski of Alaska, Susan Collins of Maine and Thom Tillis of North Carolina – will join Democrats in voting to disapprove the President’s declaration, which would grant him access to billions of dollars to spend towards border security that Congress refused to appropriate. In the House, 13 Republicans voted with Democrats to pass the resolution.

During the recent 35-day partial government shutdown, the longest federal shutdown in US history, Trump pushed Congress to spend $5.7 billion for physical barriers along the border with Mexico. The divided legislature rejected his proposal and the President eventually accepted its bill for $1.375 billion in border security measures. Before signing it, Trump announced he would go around Congress to get the wall he wanted, presenting Republicans on Capitol Hill with a difficult choice.

Many Republican senators have not yet said how they’ll vote, including Sens. Lamar Alexander of Tennessee, Cory Gardner of Colorado, Marco Rubio of Florida, Mitt Romney of Utah, Pat Toomey of Pennsylvania, Mike Lee of Utah, Martha McSally of Arizona, Ben Sasse of Nebraska, Rob Portman of Ohio, Roger Wicker of Mississippi, Ted Cruz of Texas, Roy Blunt of Missouri, James Lankford of Oklahoma and Jerry Moran and Pat Roberts of Kansas.

“I think a great many Republicans are like me,” Alexander said. “They’d like to support what the President is trying to do on border security, but we have an oath to the Constitution that we have to respect too.”

Homeland Security Department Secretary Kirstjen Nielsen gave a presentation to Republican senators urging them to support the President. Rubio said the administration cited a Reagan-era precedent that he would need to further review.

“There was some discussion of that but it wasn’t very persuasive,” said Blunt, a member of Senate GOP leadership.

The undecided Republicans represent a mix of moderates and hard-right conservatives, those retiring and those up for re-election, rank-and-file members and their leaders and those worried that the President will divert funding from military construction projects in their states to build the wall at the border. Some Republicans have said they’re worried that Trump’s actions will set an undesirable precedent for a future Democratic president to declare a national emergency on climate change or guns.

Senate Majority Leader Mitch McConnell had hoped the President wouldn’t announce a national emergency, but has supported the President’s declaration since he made it. McConnell predicted earlier this week that the Senate will vote to block the resolution, Trump will veto it and then the House will fail to get the two-thirds majority necessary to override the President.

Should Trump’s veto succeed, Congress could look for other ways of reining in the executive branch.

Lee, for example, is talking to other senators about introducing legislation to overhaul the National Emergencies Act of 1976, which the administration believes gives Trump the requisite authority but plaintiffs in various cases have challenged in court.

The bill would limit a presidential emergency declaration to 30 days, after which “affirmative” congressional action would be required to extend it, according to a Senate aide granted anonymity to discuss a bill that has not been finalized yet.

Here’s a list of 15 Republican senators to watch:

Lamar Alexander of Tennessee

Alexander has implored President Trump to find other pots of money available to him to build a border wall with Mexico and avoid going forward with his national emergency declaration.

“What I would like (Trump) to do is take another look at his existing authority and if he has sufficient funds to build 234 miles of wall, to use that,” Alexander told reporters in February. “From his point of view, it should avoid litigation, make sure the wall gets built, and avoid this dangerous precedent that I believe is unnecessary.”

“We’ve never had a case where a president has asked for money, being refused the money by Congress, then use the national emergency powers to spend it anyway,” Alexander added. “To me that’s a dangerous precedent.”

Mitt Romney of Utah

“I do not believe declaring a national emergency is the right approach,” Romney said in February, according to the Associated Press.

On Tuesday, Romney told CNN he’s “not ready” to say how he’s going to vote, but that his announcement would come “soon.”

Martha McSally of Arizona

McSally recently told reporters that she is undecided. “Still looking at it,” she said Tuesday.

Late last month, she tweeted that she had talked to Vice President Mike Pence about the national emergency declaration. “I am seeking assurances that the money will not come from Arizona military construction projects,” she said. “We can & must secure our border while ensuring our armed forces have the resources and facilities they need.”

Rob Portman of Ohio

“I’m trying to come up with an alternative way to deal with this,” Portman told reporters. “I’ll make a decision next week, of course, when the issue comes before us, but I’m trying to get a result here. I know some in the media are very eager to see an immediate decision, but that’s not the way I look at this.”

Portman added that the current declaration could set a “bad precedent” and that some of the President’s funding will be “tied up in court.”

Pat Toomey of Pennsylvania

Toomey told radio host RJ Harris on Monday that he’s looking for a way with the White House to “secure the border, but not actually upset the constitutional order of things.”

Roger Wicker of Mississippi

“I would be inclined to vote for the resolution of disapproval,” Wicker told HuffPost in February, before Trump declared the emergency. “I think it’s bad law and I think it’s bad strategy for the President.”

Ted Cruz of Texas

In February, Cruz told The New York Times, “I am very worried about the slippery slope that could occur.”

Asked by CNN how he’d vote on Tuesday, Cruz said, “I’m continuing to review the statutory authorities and arguments put forward by the administration and we continue to have vigorous and robust discussions within the conference.”

“The conversations I expect will continue right up until the vote,” he added.

Marco Rubio of Florida

When asked by CNN if he has decided how he plans to vote, Rubio recently said “No.” But in February, Rubio said, “no crisis justifies violating the Constitution,” indicating he would oppose Trump’s declaration.

“Today’s national emergency is border security,” said Rubio in a statement from February 14. “But a future president may use this exact same tactic to impose the Green New Deal. I will wait to see what statutory or constitutional power the President relies on to justify such a declaration before making any definitive statement. But I am skeptical it will be something I can support.”

Jerry Moran of Kansas

Moran recently told CNN that he’s undecided. In February, Moran said, “I’m worried that if it gets used this time, what’s the next instance in which it becomes used?”

Pat Roberts of Kansas

Last month, Roberts said he was worried about a future “government by fiat.” On Tuesday, Roberts told the AP jokingly, “I’m never undecided but I just haven’t made my mind up.”

Ben Sasse of Nebraska

“We absolutely have a crisis at the border, but as a Constitutional conservative I don’t want a future Democratic President unilaterally rewriting gun laws or climate policy,” Sasse told National Review in February. “If we get used to presidents just declaring an emergency any time they can’t get what they want from Congress, it will be almost impossible to go back to a Constitutional system of checks and balances. Over the past decades, the legislative branch has given away too much power and the executive branch has taken too much power.”

Roy Blunt of Missouri

Blunt, member of Senate GOP leadership, said in a recent interview that he still hasn’t decided exactly how he’s going to vote “because I don’t know exactly what are options are.”

“We’d still like to see the President rethink this,” he added.

Cory Gardner of Colorado

Gardner said on Tuesday he’s still trying to understand the “ramifications and legal implications” at stake. “We continue to talk to Coloradans and look into the law,” Sen. Gardner told CNN.

Mike Lee of Utah

In February, Lee tweeted, “My initial assessment is that what Pres. Trump announced is legal. Whether or not it should be legal is a different matter. Congress has been ceding far too much power to the exec. branch for decades. We should use this moment as an opportunity to start taking that power back.”

James Lankford of Oklahoma

Lankford said Tuesday that he’s looking for more information from the administration about how the national emergency declaration would move Department of Defense funding to build the border wall before announcing how he’d vote.

CNN’s Clare Foran, Phil Mattingly, Ted Barrett and Liz Landers contributed to this report.