Even before President Trump announced Friday that he was declaring a national emergency to fund a border wall, U.S. Representative Joaquin Castro, D-San Antonio, tweeted that he would lead an effort to block it.

“If @realdonaldtrump declares a national emergency to fund his border wall, I’m prepared to introduce a resolution to terminate the President’s emergency declaration under 50 U.S.C. 1622. (National Emergencies Act) #FakeEmergency

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Immediately, U.S. Representative Vicente Gonzalez, D-McAllen, whose district would be greatly affected by a wall that is being targeted for the Rio Grande Valley, posted on Twitter that he would join Castro’s effort. “I thank my colleague, @JoaquinCastrotx, for his diligence and duty to protect our lands. I will support him in this endeavor. #FakeEmergency

By Friday afternoon, Castro had over 60 co-sponsors for the resolution that goes to the heart of a Constitutional face-off between the nation’s executive and legislative branches of government.

“The President is declaring an emergency over a crisis that does not exist,” said Castro. “This unfounded declaration would take money away from actual, identified national security needs.” The San Antonio lawmaker described the effort as “an end run around the Constitution” that disrupts the balance of power between legislative and executive branches.

“The joint resolution I am introducing, along with my cosponsors, will allow Congress an opportunity to reclaim its authority and jurisdiction under the Constitution,” he said.

Texas Democrats who split the vote on the border funding package that gave Trump $1.3 billion for the border wall were looking to stop him from ratcheting up the amount to $8 billion for the wall. Trump said that he was planning on taking funding from other accounts, such as military construction.

The compromise funding law signed by the president Friday includes funding for an additional 55 miles of a border barrier in the RGV. By declaring an emergency, Trump is looking to shuffle more than $6 billion from other parts of the budget to build over 200 miles of border fencing or barrier. There is already about 650 miles of fencing along the southern border—much of it in Arizona and New Mexico.

Six Texas Democrats voted no on the funding bill—Lloyd Doggett of Austin, Veronica Escobar of El Paso, Sylvia Garcia of Houston, Filemon Vela of Brownsville, Castro, and Gonzalez—out of a total of thirteen Texas Democrats. U.S. Representative Henry Cuellar, D-Laredo, who helped craft the funding package as a member of House-Senate conference committee, said Friday that he was strongly opposed to the president’s use of emergency powers. “The president’s declaration of this ‘so-called emergency’ is a violation of constitutional authority to reallocate billions of federal dollars that have been already appropriated by the U.S. Congress,” said Cuellar.

Although Cuellar was able to secure language in the funding bill that protected certain nature habitats and a state park in South Texas, it was still unclear whether Trump’s emergency declaration would allow the president to circumvent those protections as well.

Doggett, an outspoken former Texas Supreme Court justice, said, “Trump should include the entire unnecessary wall in any phony emergency decree that he orders. Congress should immediately challenge his declaration, not let him claim our blessing for 55 miles of havoc through the Rio Grande Valley.”

Meanwhile, the state’s two senators were split on both the funding bill and the use of emergency powers. Senator Ted Cruz, who voted against the bill, said before Trump announced his plan to use emergency power, “Democrats’ intransigence has left the president with no other choice but to take executive action.”

“I am a constitutionalist, and I have long advocated that every president, Republican or Democrat, be bound by the Constitution and federal laws. I will wait to see what specific course of action the president pursues and what legal authorities are cited as a basis for it,” he said.

But Senator John Cornyn, who voted for the bill and is also a former Texas Supreme Court justice, held back from endorsing executive action, telling reporters that he needed “to learn more about what Trump will do unilaterally.” But that did not stop Cornyn, who is running for re-election next year, from sending out a fundraising email on Friday asking people to show their support for Trump’s action by sending Cornyn money. “Friend, President Trump is standing in defence [sic] of the millions of Texans placed at risk by a porous border, slowing the flow of drugs and crime into our neighborhoods,” Cornyn’s email said. “By committing to a more secure border, the President shows us that he is dead serious about protecting Americans, and he needs to know that his efforts are appreciated. Will you add your name to show your support?”

Castro, who is an attorney, cited the law that gives the president broad emergency powers but which also gives Congress the ability to terminate an emergency action through a joint resolution approved by the House and Senate. The resolution may gain support in the Democratically-controlled House but the outlook is uncertain in the GOP-controlled Senate, despite several Republicans in that chamber who have voiced concern with Trump’s declaration. It is one of the responses, including court action, that Democrats are thinking of taking in what may prove to be Trump’s biggest fight in his presidency thus far.

For Cuellar and for other border lawmakers, it’s personal. “I, along with many of my colleagues, disagree with the president’s mischaracterization of the border. The border is a place of opportunity, community, and family. I live at the border—I eat the food, breathe the air, and call it home.”

Escobar, who represents El Paso, said “President Trump’s national emergency declaration is a political stunt in order to further his administration’s anti-immigrant agenda and deliver on a campaign promise to build a wasteful and hateful border wall. President Trump’s abuse of power and lies against our southern border will be challenged in Congress and in the courts,” she said.

The lone Republican who represents the border, U.S. Representative Will Hurd of Helotes, who voted for the bill appeared on ABC’s “The View” on Friday and was asked how landowners on the border would react to the wall, said: “They’re going to sue because in the great state of Texas, we care about a little thing called ‘private property rights,'” he said.