Donald Trump acknowledged that his order would face a lengthy legal fight.

President Donald Trump on Friday declared a national emergency in a bid to fund his promised wall at the U.S.-Mexico border without congressional approval, an action Democrats vowed to challenge as a violation of the U.S. Constitution.

The Republican president's move circumventing Congress represented a new approach to a 2016 presidential campaign pledge to halt the flow of immigrants into the country, which Trump maintains spreads crime and drugs.

He was also expected later on Friday to sign a bipartisan government spending bill Congress approved on Thursday that would prevent another partial government shutdown by funding several agencies that otherwise would have closed on Saturday.

Trump made no direct mention in his Rose Garden comments of the funding bill, which represents a legislative defeat for him since it contains no money for his proposed wall - the focus of weeks of conflict between him and Democrats in Congress.

Trump's demand that Congress provide him with $5.7 billion in wall funding as part of legislation to fund the agencies triggered a historic, 35-day December-January government shutdown that damaged the U.S. economy and his opinion poll numbers.

By reorienting his wall-funding quest toward a legally uncertain strategy based on declaring a national emergency, Trump now risks plunging into a lengthy battle with Democrats and dividing his fellow Republicans.

Fifteen Democrats in the Republican-controlled Senate introduced legislation on Thursday to prevent the transfer of funds from accounts Trump likely would target to pay for the wall.

Nancy Pelosi, the Democratic speaker of the House of Representatives, and Chuck Schumer, the Senate's top Democrat, swiftly responded to Trump's declaration.

"The president's actions clearly violate the Congress's exclusive power of the purse, which our Founders enshrined in the Constitution," they said in a statement. "The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available."

New York state's attorney general, Letitia James, said her office would also challenge Trump in court.

"We won't stand for this abuse of power & will fight back with every legal tool at our disposal," James wrote on Twitter.

The president acknowledged that his order would face a lengthy legal fight. "We'll win in the Supreme Court," Trump predicted.

Legal experts said the Trump administration may prevail.

The National Emergencies Act of 1976 gives the president broad leeway to declare an emergency. The law also includes a mechanism for Congress to oppose such a declaration.

If Congress fails to vote down Trump's declaration of an emergency, said law professors Jonathan Turley of Georgetown University and William Banks of Syracuse University, courts will be reluctant to substitute their national security judgment for those of Congress and the president.