(Adds Trump, Leahy, Schumer, professor remarks, details)
WASHINGTON, Feb 15 (Reuters) - 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 to circumvent Congress represented a new approach to making good on a 2016 presidential campaign pledge to halt the flow of undocumented immigrants into the country, whom Trump says bring 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 rambling Rose Garden comments of the funding bill. It 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 had demanded that Congress provide him with $5.7 billion in wall funding as part of legislation to fund the agencies. That triggered a historic, 35-day December-January government shutdown that hurt the U.S. economy and his opinion poll numbers.
By reorienting his quest for wall funding toward a legally uncertain strategy based on declaring a national emergency, Trump risks plunging into a lengthy legislative and legal battle with Democrats and dividing his fellow Republicans.
Fifteen Democrats in the Republican-controlled Senate introduced legislation on Thursday to prevent Trump from invoking emergency powers to transfer funds to his wall from accounts Congress has already committed to other projects.
Nancy Pelosi, the Democratic speaker of the House of Representatives, and top Senate Democrat Chuck Schumer swiftly responded to Trump's declaration.
"The president's actions clearly violate the Congresss 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 court fight.
"I expect to be sued. I shouldn't be sued ... We'll win in the Supreme Court," Trump predicted.
Both the House and the Senate could pass a resolution terminating the emergency by majority vote. However, that measure would then go to Trump, who would likely veto it. Overriding the veto would require a two-thirds vote in both chambers.
Trump says a wall is needed to curb illegal immigrants and illicit drugs coming across the border. But statistics show that illegal immigration via the border is at a 20-year low and that many drug shipments come through legal ports of entry.
Confronted with those statistics at the Rose Garden event with reporters, Trump said they were "wrong."
Also present were a half-dozen women holding poster-sized pictures of family members killed by illegal immigrants. Trump cited their presence in announcing the emergency declaration.
Trump estimated his emergency declaration could free up as much as $8 billion to pay for part of the wall. Estimates of its total cost run as high as $23 billion.
As a candidate, Trump repeatedly promised Mexico would pay for the wall. It was one of his biggest applause lines at his campaign rallies. Mexico firmly refused to pay, and now Trump wants U.S. taxpayers to cover the costs.
Under the Constitution, major commitments of taxpayer funds are controlled by Congress, not the White House. After two years of legislative debate, Congress has refused to fund the project.
Senator Patrick Leahy, a Democrat, appealed to Senate Republicans to resist Trump's emergency declaration.
"Lets not forever change the course of the separation of powers in our country simply because the president made a foolish promise he could not keep," he said in a statement.
Democrats said the president had fabricated a "crisis" at the border to justify an unconstitutional grab for funding.
In his remarks on the emergency declaration, Trump said, "I didn't need to do this. But I'd rather do it much faster."
Afterward, Schumer on Twitter asked, "Mr. President, how can this possibly be a national emergency if youre saying you dont need to do it?"
With an emergency formally declared, Trump was expected to fly later on Friday to his Mar-a-Lago golf resort in Florida for a holiday break.
(Reporting by Richard Cowan and Roberta Rampton; additional reporting by David Morgan, Steve Holland, Susan Cornwell, Makini Brice, Alison Frankel and Eric Beech; writing by James Oliphant, Kevin Drawbaugh and Peter Cooney; editing by Jeffrey Benkoe and Jonathan Oatis)