Washington(ANN)-US President Donald Trump’s decision to declare a national emergency to fund his border wall has already been hit with the first lawsuit, in what is expected to become a lengthy legal face-off between the Republican head of state and his opponents over the controversial barrier.

Liberal advocacy group Public Citizen filed the lawsuit on behalf of the Frontera Audubon Society and three landowners in South Texas who had been informed their land would be used to build the wall.

The lawsuit sought to block Trump’s declaration, which deferred $8.1 billion from military projects for the wall.

Trump’s move came after Democrats in Congress refused to give him $5.7 billion for the wall on US-Mexico border. The showdown resulted in the longest ever partial shutdown of the US government which began on December 22 and lasted for 35 days.

To avert a second shutdown, Trump on Wednesday signed a new spending bill that allocated $1.3 billion for border security but did not give him any money for the wall.

Public citizen warned that Trump’s declaration was in violation of the US constitution and undermined the separation of powers in America.

“The complaint urges the court to find that Trump exceeded his constitutional authority and authority under the National Emergencies Act, and to hold that the declaration violates the doctrine of separation of powers that is so central to our Constitution,” the group said in a press release.

“The court should bar Trump and the US Department of Defense from using the declaration and funds appropriated for other purposes to build a border wall, the complaint requests,” it asserted.

Upon signing his national emergency directive, Trump predicted that he was going to face many legal challenges.

“We will possibly get a bad ruling, and then we’ll get another bad ruling and then we’ll end up in the Supreme Court,” Trump said, expressing confidence that would prevail at the high court where conservative judges hold a 5-4 majority.

“I didn’t need to do this, but I’d rather do it much faster,” he added, referring to the construction of the wall, a central promise of his 2016 presidential campaign.

Democrats have already announced their intention to file similar lawsuits, disputing Trump’s claims of an ‘invasion’ of drugs and criminals at the southern border.

California Governor Gavin Newsom, a Democrat, said Friday the he was going to sue the administration over Trump’s decision.

“President Trump is manufacturing a crisis and declaring a made-up ‘national emergency’ in order to seize power and subvert the constitution,” Newsom said in a statement. “Our message back to the White House is simple and clear: California will see you in court.”

Democrats on the House Judiciary Committee have also pledged to launch an investigation and grill the White House counsel and Justice Department officials, arguing that declaration showed “disregard” for the separation of powers and Congress’ authority over the federal budget.

Backlash continues

The decision also drew fire from business and conservative groups typically aligned with the Republican Party.

The US Chamber of Commerce, FreedomWorks advocacy group and the Heritage Foundation, all warned that the decision sets a precedent for radical actions by Trump’s successors.

“The declaration of national emergency in this instance will create a dangerous precedent that erodes the very system of government that has served us so well for over 200 years,” said Thomas J. Donohue, who presides over the US Chamber of Commerce.

Adam Brandon, president of FreedomWorks, said the decision gave the executive branch too much power and undermined Congress’ influence over spending.

“No matter whether a Republican or a Democrat sits in the Oval Office, the concentration of power in the executive branch is alarming,” he said.

Heritage Foundation President Kay Coles James warned that the move “exposes the critical need for border security to the whims of activist federal judges.”