The U.S. House of Representatives is set to vote Tuesday on a resolution aimed at overturning the national emergency declaration issued earlier this month by President Donald Trump.

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.

“This isn’t about the border,” House Speaker Nancy Pelosi said Monday at a news conference. “This is about the Constitution of the United States. This is not about politics. It’s not about partisanship. It’s about patriotism.”

The Democratic-controlled House is expected to pass the one-page resolution and send it to the Republican-held Senate, where it would take only a handful of GOP defections to pass it. If the resolution passes, it's likely to trigger a veto from Trump, the first of his presidency, according to The Associated Press.

If the measure passes with a two-thirds majority it would be protected against a presidential veto, but House Minority Leader Kevin McCarthy, R-California, told CNN on Monday that he was confident the measure would fail to reach that bar.

In a tweet posted Monday, Trump urged Republicans to stick with him and vote against the resolution.

“I hope our great Republican Senators don’t get led down the path of weak and ineffective Border Security,” the president 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!”

Still, some GOP lawmakers expressed reservations about Trump's declaration, which came after a record 35-day partial government shutdown was triggered by the battle over border wall funding. Sen. Susan Collins, R-Maine, Sen. Lisa Murkowski, R-Alaska, and Sen. Thom Tillis, R-North Carolina, have indicated they plan to vote for the resolution, The Washington Post reported.

"It is my responsibility to be a steward of the Article I branch, to preserve the separation of powers and to curb the kind of executive overreach that Congress has allowed to fester for the better part of the past century," Tillis wrote in an opinion piece published Monday by the Post. "I stood by that principle in the Obama administration, and I stand by it now."

Democrats have argued that despite Trump’s claims, no emergency exists on the border, where crossings are down to a nearly 40-year low.

Dozens of former national security experts, including former secretaries of state Madeleine Albright and John Kerry and former defense secretaries Chuck Hagel and Leon Panetta, issued a public declaration Monday in which they said they were "aware of no emergency that remotely justifies" the emergency declaration, Politico reported.

"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," the letter says, according to The Washington Post.

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-Indiana, once the chairman of the Senate Foreign Relations Committee.

The president’s emergency declaration, announced after Congress declined to give the president the $5.7 billion he requested to fund the building of a wall on the U.S.-Mexico border, will funnel over $6 billion in funds from the Treasury Department and the Pentagon to pay for the wall.

The shifted funds would include $3.6 billion earmarked for already approved military construction projects, according to Cox Media Group's Jamie Dupree.

The Associated Press contributed to this report.