House Democrats on Tuesday are poised to pass legislation blocking President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s emergency declaration on the southern border, sending a clear rebuke to the president and his go-it-alone approach to border security.

The resolution, sponsored by Rep. Joaquin Castro Joaquin CastroHispanic Caucus members embark on 'virtual bus tour' with Biden campaign Hispanic caucus report takes stock of accomplishments with eye toward 2021 Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-Texas), is unlikely to survive Trump’s promised veto, even if Senate Democrats can find enough GOP support in the upper chamber to move it to the president’s desk in the coming weeks.

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But by bringing the measure quickly to the floor, Democratic leaders are sowing divisions between Republican lawmakers and their party’s standard-bearer.

They are also sending an unmistakable signal to the White House that, after two years dealing with a friendly, Republican-controlled Congress, Trump and his favored policy agenda will face fierce headwinds heading into the 2020 election under a newly divided government.

Democrats have hammered Trump’s emergency declaration as an illegal “power grab,” arguing that he’s shifting funds in violation of the clear-cut separation of powers outlined by the Constitution, which grants Congress the sole authority to direct where taxpayer dollars are spent.

“The president’s power grab usurps that constitutional responsibility, and fundamentally violates the balance of power envisioned by our Founders,” Speaker Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE (D-Calif.) said Monday in the Capitol.

“This isn’t about the border, this is about the Constitution of the United States.”

Castro’s resolution, just one page long, would kill Trump’s emergency order and prevent the administration from moving funds from other programs — including $3.6 billion in military construction projects proposed by Trump — to expand border wall construction.

Trump had promised a “big, beautiful” wall during his 2016 campaign, and has grown increasingly frustrated that Congress had thwarted his demands to fund it.

The House is expected to pass the Castro resolution easily on Tuesday, in a vote that’s likely to fall largely along partisan lines.

The measure was endorsed by only one Republican, Rep. Justin Amash Justin AmashCentrist Democrats 'strongly considering' discharge petition on GOP PPP bill On The Trail: How Nancy Pelosi could improbably become president History is on Edward Snowden's side: Now it's time to give him a full pardon MORE (Mich.), who frequently breaks with his party on issues related to constitutional powers.

And despite some grumbling from a number of GOP lawmakers — particularly over the shift of money out of the Pentagon — Democrats say they expect no more than a handful of additional Republicans to join them in formally rebuking Trump’s power play.

One Democratic aide put the estimated number of GOP defections at between five and 10, and GOP leaders have expectations for a figure in the same ballpark.

“I think there will be more GOP defections in the Senate than in the House,” a House GOP lawmaker close to leadership said Monday.

Politically, there is little incentive for House Republicans to buck Trump, particularly on his signature issue of border security, which remains an energizing issue in deep-red districts.

The emergency declaration, furthermore, is highly popular among Republican voters.

And even those Republican lawmakers voicing concerns over the precedent-setting nature of Trump’s action appear to be lining up in defense of their ally in the White House.

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“I would say it’s a national emergency,” Rep.(R-Wis.) told CNN Monday.

“But I do get concerned,” he continued. “Will a President Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE say global warming is a national emergency? And will, you know, will someone else say, well, the guns are a national emergency?”

In the Senate, the GOP opposition to Trump’s unilateral move has been more pronounced.

Sen. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-Maine), who’s facing a tough 2020 reelection fight, has already said she’ll support the disapproval resolution when it comes over from the House.

And Sen. Lisa Murkowski Lisa Ann MurkowskiSenate GOP aims to confirm Trump court pick by Oct. 29: report Senate GOP sees early Supreme Court vote as political booster shot Pence defends Trump's 'obligation' to nominate new Supreme Court justice MORE (R-Alaska) told a local Alaska TV station that she’s “likely” to back it as well.

The measure is deemed “privileged” under a 43-year-old law, meaning it’s guaranteed a vote in the upper chamber despite Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE’s (R-Ky.) vocal support for Trump’s emergency order.

Additionally, the disapproval resolution will require only a simple majority to pass the Senate, meaning just four Republicans — including Collins and Murkowski — would have to join Democrats to send the measure to Trump.

Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.), a close Trump ally who supports the emergency declaration, predicted last week that the disapproval resolution will pass both chambers.

Trump, for his part, has promised to veto the measure if it gets that far.

“Will I veto it? 100 percent. 100 percent,” he said Friday. “And I don’t think it survives a veto.”

Trump announced the emergency declaration on Feb. 15, a day after Congress passed an enormous spending bill to prevent a second government shutdown.

That package included $1.375 billion for border barriers, but nothing close to the $5.7 billion for new wall construction that Trump had initially demanded.

The emergency order will allow him to tap funds earmarked for other programs to expand the construction effort, without explicit approval from Congress.

Trump has said the physical barrier is vital if U.S. law enforcers are to be successful stemming the flow of drugs and violent crime across the border — a message he amplified on Monday.

“To protect our communities, we must secure the border against human trafficking, drug smuggling and crime of all types,” Trump said from the White House, where he was hosting a breakfast with state governors.

“We have an emergency of people pouring into our country that we don’t want — criminals, smugglers. We have drugs pouring into our country, we can’t have it. We can’t have it, we cannot allow this to happen to our country.”

Democrats reject Trump’s depiction of a crime-infested border, citing statistics from Trump’s own border officials in arguing against the national emergency designation.

Aside from Castro’s resolution, which is not expected to have enough support in either chamber to override a Trump veto, Democrats are also weighing their legal options.

“We would be delinquent in our duties if we did not resist, if we did not fight back, to overturn the president’s declaration,” Pelosi said. “To not do that would be to abandon our own responsibilities.

“We do not intend to do that.”

Scott Wong contributed.