U.S. House Speaker Nancy Pelosi used the backdrop of a border bridge in Laredo on Friday to open her push to block President Trump's national emergency declaration, which Democrats say could re-direct nearly $520 million in military construction funds from Texas.

"The president's act is lawless and does violence to our Constitution, and therefore, to our democracy," Pelosi told reporters. "Not only is he disrespecting the legislative branch and the Constitution of the United States, he is dishonoring the office in which he serves."

Pelosi was joined by Laredo Mayor Pete Saenz and U.S. Rep. Henry Cuellar, a Laredo Democrat who helped negotiate a bipartisan $1.37 billion border funding deal that averted another partial government shutdown. Trump, however, has sought some $6 billion for new border barriers and has vowed to use emergency powers to circumvent Congress and siphon money from the Pentagon.

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The Democratic-led House plans to vote Tuesday to pass legislation opposing Trump's move, which critics say is unconstitutional. The resolution, brought by San Antonio Democrat Joaquin Castro, would then force a vote within 18 days in the Senate, where some Republicans have signaled their support.

"This is a historic power grab, and it will require historic unity by members of Congress, Republican and Democrat, liberal and conservative, to counteract the president's parasitic movement," Castro, chairman of the Congressional Hispanic Caucus, told reporters in a conference call Friday.

Democrats have also argued that there is no "crisis" at the border, noting that border apprehensions have decreased from previous decades. A group of 16 states also filed suit to block the emergency declaration.

Trump said later Friday that he would veto any measure from Congress to block his emergency declaration.

Cuellar, a member of the House Appropriations Committee, released a list earlier this week of what he said are potential cuts in Texas, including $251 million to replace an Air Force hospital at Fort Bliss. The other threatened projects include construction of an Air Force dormitories, classrooms and dining facilities at Joint Base San Antonio.

Cuellar also identified potential cuts to programs at Fort Hood and Red River.

"After failing to convince the Government of Mexico or the U.S. Congress to pay for his ineffective wall, the president is trying to bypass constitutional authority with his 'emergency' declaration," Cuellar said. "He is taking duly appropriated funding intended for the military, law enforcement, and many other priority areas in order to follow through on his campaign promise."

Pelosi accused Trump of trying to "usurp the power of Congress" to spend money. "Those in the military don't want the money taken out of their defense budget for a purpose that is clearly not a defense purpose," she said.

The White House has disputed the House Democrats' list, which includes billions of dollars in unspent funds for military construction projects and programs across the nation. A senior administration official, speaking on condition of anonymity, said the White House is working with the Defense Department to identify sources of wall funding, but that there is no list of planned projects that might be affected "at this time."

The official said the criteria will focus on lower priority, "un-awarded projects" without harming "lethality or readiness."

Administration officials also have signaled that they will first tap a Treasury Department forfeiture and Pentagon counterdrug funds before they turn to military construction projects.

The office of Texas Gov. Greg Abbott declined to comment publicly on the potential cuts in Texas, though privately state officials say they have no reason to believe any military bases in the Lone Star State will lose money.

Amid legal challenges to Trump's move, it remains unclear what, if any, congressionally-earmarked funds could be vulnerable to the president's edict at some point. Trump and many legal analysts have predicted that the lawsuits could be sent up to the U.S. Supreme Court, a process that could go on beyond the 2020 election.

Though some Texas Republicans in Congress have expressed misgivings about the president's use of emergency powers to obtain wall funding, they have sought to place the blame for the impasse on Democrats.

"Democrats gave the President no leeway to enact his expert-led border security plan, so I share his frustration with the position we are in now," said Houston Republican Dan Crenshaw, a former Navy SEAL. "While I'm hopeful that this option will start to address the problems at our border, I remain wary of the precedent it sets. This is simply the result of Congress not doing its job."

Texas U.S. Sen. John Cornyn, speaking recently in San Antonio, suggested that Trump is making use of powers granted to him by Congress, a formulation that some Republicans believe would not violate the separation of powers doctrine in the U.S. Constitution, which gives Congress the power of the purse.

"I think the president was frustrated that he didn't get everything he wanted from Congress, and he's using existing authorities that Congress has given him to get access to reprogram other funds," Cornyn said. "I regret that he thought that was necessary because we should be working together in a bipartisan basis to try to provide border security."

Cornyn and Senate GOP Leader Mitch McConnell had previously sought to discourage the president from pursuing wall funds using emergency powers, which some say could set a precedent for future Democratic presidents to employ for gun control, climate change or health care.

Under pressure, many Republicans in Texas and elsewhere have maintained a silence about Trump's funding maneuver.

Texas U.S. Sen. Ted Cruz said after Trump's emergency declaration that he would want to see what the president's specific course of action would be, and what legal authorities he would cite. Cruz's Senate office did not respond to multiple requests for comment this week.