U.S. Sen. Lamar Alexander, R-Tenn., speaks at the unveiling of the official portrait of Tennessee Gov. Bill Haslam, Dec. 17, 2018, in Nashville, Tennessee.
U.S. Sen. Lamar Alexander, R-Tenn., speaks at the unveiling of the official portrait of Tennessee Gov. Bill Haslam, Dec. 17, 2018, in Nashville, Tennessee.

WASHINGTON - Senate opponents of President Donald Trump's declaration of a national emergency at the Mexican border moved within a hair Thursday of having enough votes to prevail, and one Republican suggested the president could face a rejection by the GOP-led chamber if he didn't change course.

Trump's move would ``turn a border crisis into a constitutional crisis,'' veteran Sen. Lamar Alexander of Tennessee said on the Senate floor. But he stopped short of saying he'd support a resolution blocking the president's move. Had Alexander pledged his vote, opponents of the emergency would probably have enough votes to pass a measure repealing the declaration.

Speaking later to reporters, Alexander warned about what might happen if Trump doesn't settle for using other money he can access without declaring an emergency.

``He can build a wall and avoid a dangerous precedent, and I hope he'll do that,'' Alexander said. ``So that would change the voting situation if he would agree to do that.''

The Democratic-led House voted Tuesday to upend Trump's move, which if left standing would let him circumvent Congress and funnel billions of extra dollars to build his proposed wall. The Senate is considering the measure.

FILE - People gather during a protest against U.S.
FILE - People protest President Donald Trump's declaration of a national emergency to build a border wall, outside Trump International Hotel & Tower in Manhattan, N.Y., Feb. 15, 2019.

?Optional language

Exactly what the Senate will vote on remains unclear. Several Republicans said that behind closed doors, they were considering several options for alternative language, including making it harder for future presidents to divert federal dollars to projects of their choosing by declaring emergencies.

Republicans say a Senate vote is likely in two weeks.  

Trump has promised to veto the House-passed resolution. Congress appears all but certain to lack the two-thirds majorities in each chamber that would be needed to override his veto, but the showdown carries risks for GOP lawmakers.

Underscoring that, Trump warned Republicans against challenging him. While the wall and other Trump moves curbing immigration elicit wide public opposition, he remains wildly popular with hard-right voters, and GOP lawmakers cross him at their peril.

"I really think that Republicans that vote against border security and the wall — I think you know I've been OK at predicting things — I think they put themselves at great jeopardy,'' Trump said in excerpts of an interview with Fox News Channel's Hannity show released Thursday.

President Donald Trump speaks during an event in t
FILE - President Donald Trump, pictured in the White House Rose Garden, says he'll declare a national emergency in order to build a wall along the southern U.S. border, Feb. 15, 2019, in Washington.

?Precedent feared

While congressional Republicans are reluctant to challenge Trump, many say his move tramples Congress' constitutional power to control spending. They also say it would set a precedent for future Democratic presidents to declare emergencies for their own purposes, and they worry that he would siphon money for barrier construction from projects in their states.

Alexander, a three-term senator who will retire in 2021 and is known for reaching across the aisle, has no re-election worries. 

"I support what the president wants to do on border security, but not the way he has been advised to do it,'' said Alexander, 78. ``It is unnecessary and unwise to turn a border crisis into a constitutional crisis about separation of powers.'' 

Presidents have declared 58 national emergencies under a 1976 law. But never has one declared an emergency after Congress had explicitly denied the money in question. 

GOP Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and Thom Tillis of North Carolina have said they will vote to derail the emergency declaration. Just four would be needed to send the measure to Trump for his promised veto, assuming that all 47 Democrats and their independent allies vote as expected to confront the president.

No filibuster possible

By law, the resolution cannot be filibustered and would need just a majority of 51 votes to pass the Senate.

Congress has voted to limit spending for barriers to just under $1.4 billion. Trump featured the wall as a central plank of his presidential campaign and repeatedly said Mexico would pay for it, which hasn't happened.

Trump has said he needs additional barriers to halt drugs, human traffickers and unauthorized immigrants from slipping into the U.S. Opponents say there is no crisis.

Border Patrol agent Vincent Pirro walks near where
FILE - Border Patrol agent Vincent Pirro walks near where the border wall ends that separates Tijuana, Mexico, left, from San Diego, right, Feb. 5, 2019.

The emergency declaration would let Trump divert $3.6 billion from military construction to erect more border barriers. He's invoking other authorities to transfer an additional $3.1 billion to construction.

Lawsuits have been filed aimed at derailing the declaration, which could at least prevent Trump from getting the extra money for months or more.

Meanwhile, Collins and three other moderate senators introduced their own resolution blocking the emergency.

The proposal is identical to the one-sentence, House-approved measure. The new legislation gives the Senate and the four centrist sponsors — two Republicans and two Democrats — a chance to put their stamp on congressional opposition to Trump's move.

The other sponsors are Sens. Tom Udall, D-N.M., Jeanne Shaheen, D-N.H., and Lisa Murkowski, R-Alaska. Only Murkowski does not face re-election next year.