WASHINGTON (AP) — 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 he risks a rebuff by the GOP-led chamber if he doesn't change course.

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

Speaking later to reporters, Alexander, R-Tenn., warned 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 declaration, which he declared to circumvent Congress and funnel billions of extra dollars to erecting his proposed wall.

Trump has promised to veto the effort to thwart him, and Congress seems all but certain to lack the two-thirds majorities in each chamber that would be needed to override his veto. But the showdown puts GOP lawmakers in a ticklish spot that party leaders are hoping to ease.

Republicans say a Senate vote is likely in two weeks, but exactly what the Senate will vote on remains unclear.

Several Republicans said that behind closed doors, they were considering several options for alternative language. Sen. Mike Lee, R-Utah, was among those suggesting provisions making it harder for future presidents to divert federal dollars to projects of their choosing by declaring emergencies.

"I wouldn't be surprised if some changes are made," said Sen. John Cornyn, R-Texas.

While the wall elicits wide public opposition, Trump remains wildly popular with hard-right voters and GOP lawmakers cross him at their peril. Trump warned Republicans anew against challenging him.

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"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.

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

Alexander, a three-term senator who plans to retire in 2021, 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, Alexander said.

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 Republicans 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.

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

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.

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 lets senators 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 doesn't face re-election next year.

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AP Congressional Correspondent Lisa Mascaro in Washington and AP writer David Sharp in Portland, Maine, contributed.