The emergency declaration is expected to face an array of other legal challenges.
House Speaker Nancy Pelosi, D-Calif., and Senate Minority Leader Chuck Schumer, D-N.Y., issued a statement calling Trump's emergency declaration "unlawful."
"The President's actions clearly violate the Congress's exclusive power of the purse, which our Founders enshrined in the Constitution," the statement said. "The Congress will defend our constitutional authorities in the Congress, in the Courts, and in the public, using every remedy available."
"The President is not above the law," the statement said. "The Congress cannot let the President shred the Constitution."
Senate Majority Leader Mitch McConnell, R-Ky., said Trump's hand was forced on declaring a national emergency by congressional Democrats.
"President Trump's decision to announce emergency action is the predictable and understandable consequence of Democrats' decision to put partisan obstruction ahead of the national interest," McConnell said in a statement. "I urge my Democratic colleagues to quickly get serious, put partisanship aside, and work with the president and our homeland security experts to provide the funding needed to secure our borders as we begin the next round of appropriations."
Trump acknowledged in his remarks that his declaration of a national emergency would face court challenges and that he could lose in lower courts.
"Hopefully we'll get a fair shake" in the Supreme Court, Trump said. "We're declaring it for virtual invasion purposes."
He later added: "Sadly, we'll be sued, and sadly, we'll go through a process."
Sen. Thom Tillis, R-N.C., spoke out strongly against Trump's emergency declaration, raising the spectre of future Democratic presidents using a similar tactic to accomplish policy goals at odds with Republican positions.
"I don't believe a national emergency declaration is the solution," Tillis said in a statement. "It wouldn't provide enough funding to adequately secure our borders, it would likely get tied up in litigation, and most concerning is that it would create a new precedent that a left-wing President would undoubtedly utilize to implement their radical policy agenda while bypassing the authority of Congress."
California Gov. Gavin Newsom, a Democrat, said that his state will take Trump to court to block his emergency declaration.
"This 'emergency' is a national disgrace, and the blame lays solely at the feet of the President," Newsom said in a statement.
The governor took issue with Trump's plans to divert drug diversion funding to building barriers at the border.
"He plans to shut down and divert funds used by California law enforcement that run counter-narcotics operations and fight drug cartels to build his wall," Newsom said. "Our message back to the White House is simple and clear: California will see you in court."
New York Attorney General Letitia James, a Democrat, said in a statement that the president had created a "constitutional crisis."
"This action will harm Americans across the country by diverting funds necessary to handle real emergencies and real disasters to advance the President's personal agenda," the statement said. "We will not stand for this abuse of power and will fight back with every legal tool at our disposal."
Several Democratic presidential hopefuls for 2020 were also critical of Trump.
"Not getting what you want to fulfill a campaign promise/chant is not a national emergency," said Sen. Amy Klobuchar, D-Minn., "Taking money from real needs and emergencies is what will create an actual emergency.
Sen. Kamala D. Harris, D-Calif., meanwhile tweeted that the focus should be on "actual emergencies that plague our nation - like climate change or health care access - not playing politics in order to build a wasteful border wall."
