Democratic lawmakers, states and others mulling legal challenges to President Donald Trump's national emergency declaration to obtain funds to build a US-Mexico border wall face an uphill and probably losing battle in a showdown likely to be decided by the conservative-majority Supreme Court, legal experts said.

After being rebuffed by the US Congress in his request for $5.7 billion to help build the wall that was a signature 2016 campaign promise, Trump on Friday invoked emergency powers given to the president under a 1976 law. The move, according to the White House, enables Trump to bypass lawmakers and redirect money already appropriated by Congress for other purposes and use it for wall construction.

Peter Shane, a professor at Ohio State University's Moritz College of Law, said challenges to the emergency declaration could end up as a replay of the legal battle against Trump's travel ban targeting people from several Muslim-majority nations. The Supreme Court last year upheld the travel ban after lower courts had ruled against Trump, with the justices giving the president deference on immigration and national security issues.

Trump has painted illegal immigration and drug trafficking across the border as a national security threat.

Democrats, state attorneys general and at least one advocacy group have vowed to take the Republican president to court over the declaration.

"I'll sign the final papers as soon as I get into the Oval Office and we'll have a national emergency and then we'll be sued," Trump said at the White House.

The National Emergencies Act of 1976 has been invoked dozens of times by presidents without a single successful legal challenge. Congress never defined a national emergency in the law.

The legal experts said Trump's declaration could be challenged on at least two fronts: that there is no genuine emergency and that Trump's action overstepped his powers because under the US Constitution Congress has authority over federal appropriations, not the president.

The Supreme Court has a 5-4 conservative majority.

"The handwriting is on the wall here," said Steven Schwinn, a professor at the John Marshall Law School in Chicago. "The Supreme Court is almost certain to uphold President Trump's emergency."