WASHINGTON (AP) — Democrats are moving quickly to try to roll back President Donald Trump's declaration of a national emergency to siphon billions of dollars from the military to fund construction of a fence along the U.S.-Mexico border.

Tuesday's vote in the Democratic-controlled House comes on legislation to revoke Trump's executive order from earlier this month and would send it to the Republican-held Senate, where it would take only a handful of GOP defections to pass it.

Trump is likely to prevail in the end since he could use his first-ever veto to kill the measure if it passes Congress, but the White House is seeking to minimize defections among the president's GOP allies to avoid embarrassment.

The vote could be challenging for GOP lawmakers who view themselves as conservative protectors of the Constitution and the powers of the federal purse that are reserved for Congress. But GOP vote counters are confident that the tally won't get near the two-thirds that would overturn a Trump veto.

Democratic leaders said Monday that the vote is not about the merits of Trump's wall but how Trump is trampling on the Constitution by grabbing money that he can't obtain through normal means.

"The beauty of the Constitution, the heart and soul of the Constitution, is the separation of power — co-equal branches of government to be a balance of power," said House Speaker Nancy Pelosi, D-Calif. "The Constitution spells out the responsibilities, giving the Congress of the United States, among other powers, the power of the purse. The president's power grab usurped that constitutional responsibility and fundamentally violates the balance of power envisioned by our founders."

Minority Leader Kevin McCarthy, R-Calif., said GOP defections will be kept well below the threshold required to sustain a veto. Describing the argument GOP leaders are using to tamp down Republican opposition, he said, "There's an emergency along the border."

"If Republicans vote their beliefs, we'll get a lot. If they vote their party, we won't get a lot," said Majority Leader Steny Hoyer, D-Md.

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Trump took to Twitter on Monday to urge Senate Republicans to stick with him.

"I hope our great Republican Senators don't get led down the path of weak and ineffective Border Security," Trump wrote. "Without strong Borders, we don't have a Country — and the voters are on board with us. Be strong and smart, don't fall into the Democrats 'trap' of Open Borders and Crime!"

Vice President Mike Pence is expected to discuss the issue with GOP senators during their weekly private lunch. A Justice Department official is also expected to attend.

On Monday, GOP Sen. Thom Tillis, R-N.C., said he would vote to block the order, joining Maine's Susan Collins and Alaska's Lisa Murkowski as Republicans supporting the resolution. Congress must defend its power of the purse and warned that a future Democratic president might abuse the power to advance "radical policies," Tillis said.

Those three would put the vote at a 50-50 tie if every Democrat supports the measure, and other GOP senators oppose Trump's maneuver as well, though the White House was scrambling to stave off defections. It looks fairly certain that enough Republicans will vote in favor of the measure to put it over the top.

"I think it's unnecessary, unwise, and inconsistent with the United States Constitution and I'll decide how to vote when I'm presented with something to vote on," said Lamar Alexander, R-Tenn., who is retiring at the end of the term. "I feel strongly about it."

Senate voting on Trump's emergency order could drag under a rarely used procedure, which an aide said is possibly a first for the chamber. The law allows for up to 15 days of committee review— in this case, at the Armed Services panel — with a full Senate vote three days later. Senators, though, said the process could be expedited.

At issue is Trump's longstanding vow to build a wall along the 1,900-mile southwest border, perhaps his top campaign promise. He has long since dropped any pretense that money for the wall would come from Mexico, which he once claimed would be the source of funding.

Earlier this month Congress approved a huge spending bill providing nearly $1.4 billion to build 55 miles (89 kilometers) of border barriers in Texas' Rio Grande Valley, ending a dispute that had led to a record 35-day partial shutdown of the government. Trump had demanded $5.7 billion to construct more than 200 miles (322 kilometers).

Also Monday, national security experts and former GOP lawmakers issued public declarations against Trump's edict, saying that the situation along the southern border is not a genuine emergency and that Trump is abusing his powers.

"We are aware of no emergency that remotely justifies such a step," wrote 58 former senior national security officials, including Republican Chuck Hagel, a former Nebraska senator and defense secretary. "Under no plausible assessment of the evidence is there a national emergency today that entitles the President to tap into funds appropriated for other purposes to build a wall at the southern border."

In addition, 28 Republican former House members and senators, many of them from the party's shrinking moderate wing, wrote an open letter declaring their opposition to Trump's emergency declaration.

"How much are you willing to undermine both the Constitution and the Congress in order to advance a policy outcome that by all other legitimate means is not achievable?" wrote the former GOP lawmakers, among them former Sen. Richard Lugar, R-Ind., once the chairman of the Senate Foreign Relations Committee.

"It was a lawless act, a gross abuse of power, and an attempt to distract from the fact that he broke his core promise — to have Mexico pay for the wall," said top Senate Democrat Chuck Schumer of New York.

Trump's declaration of a national emergency gives him access to about $3.6 billion in funding for military construction projects to divert to border fencing. Lawmakers in both parties are recoiling at the politically toxic prospect of losing cherished projects at back-home military bases. The Defense Department has not identified which projects may face the ax.

But the administration is more likely to tap $600 million from a federal asset forfeiture fund first. In addition, it is considering shifting more than $2 billion from Defense Department accounts into a Pentagon counter-drug fund to be tapped for wall construction.

Trump's edict is also being challenged in the federal courts, where a host of Democratic-led states such as California are among those that have sued to overturn the order. The House may also join in.