WASHINGTON While the six previous presidents collectively declared dozens of national emergencies, Donald Trump s one over a border wall would be the first to finance an unpopular construction project that congresses in both his own country and a neighboring one have refused to pay for.

On Thursday, the White House said Trump would sign a spending bill to keep the government funded but he would also declare a national emergency since Congress wasnt going to give him the billions he wants for a wall between the United States and Mexico.

Trump has been warning for months that he would declare an emergency if Congress failed to appropriate $5.7 billion for a wall along the border with Mexico. He repeated the threat at a recent re-election rally in El Paso, Texas.

As I was walking up to the stage, they said that progress is being made with this committee, he said of a bipartisan deal that would provide $1.375 billion for border barriers. Just so you know, were building the wall anyway.

For Trump, though, choosing to declare a national emergency would likely trigger a law from Congress finding his declaration invalid , lawsuits challenging its legality, or both.

The National Emergencies Act does not give the president the authority to throw a tantrum just because Congress chooses not to give him the toys that he likes, said Greg Chen, the head of government relations for the American Immigration Lawyers Association.

Im sure the legal challenges would be filed instantly, said Elizabeth Goitein, an expert on national emergencies at the Brennan Center for Justice.

Of late, Trump and his allies have argued that national emergencies are nothing extraordinary, that every previous president going back to the passage of the National Emergencies Act in 1976 has used it and that nearly three dozen are still in force.

On Sunday he tweeted to his 58 million followers a statement from Rep. Tom McClintock (R-Calif.): There have been 58 National Emergencies declared since the law was enacted in 1976, and 31 right now that are currently active, so this is hardly unprecedented.

Yet what the White House and Trumps defenders do not mention is that no previous president has declared a national emergency for a pet project that most Americans oppose, that Congress has refused to support and that he promised during his campaign that he would force Mexico to pay for.

Douglas Rivlin, a spokesman for the pro-immigration group Americas Voice, said Trump is primarily trying to work up his hard-core base by painting immigrants as criminals, just as he did in the weeks leading up to the November midterm elections.

Rivlin added that it was ironic that an absurd campaign promise has now become the singular focus of his administration. Its always been a joke. Its always been a promise that didnt make sense, he said. It was an applause line. And yet it is driving national policy at this point.

Of course, Trump did not offhandedly joke about forcing Mexico to pay for a massive, 30-foot-tall, reinforced concrete wall. He promised that he would force Mexico to pay for it literally hundreds of times in his rallies, and even in interviews when he was asked whether he was serious.

In a March 2016 memo , Trump claimed he would force Mexico to make a one-time payment of $5-$10 billion or face garnishment of remittances from Mexican immigrants to their families in Mexico.

That fit into his brand of being this master negotiator able to make people do what they didnt want to do, Rivlin said. But even that was being pulled out of his ample presidential butt.

Upon taking office, Trump did not once broach the idea of Mexico paying in his conversations with that countrys leaders, according to the Mexican government. In fact, in his first conversation with Mexicos then-President Enrique Pea Nieto days after taking office, Trump acknowledged that Mexico would never pay for the wall but asked Pea Nieto not to say that publicly to avoid hurting Trumps standing with his supporters.

After that phone call, more than a year passed before Trump started demanding a wall, only this time from U.S. taxpayers and Congress, which led to his claims starting in October that he had the power simply to ignore Congress.

When the rules dont suit him, he breaks the rules, and then it gets tied up in courts, Rivlin said. This has been the pattern of his life.

At issue is the exclusive role given to Congress in the Constitution to decide whether and how much to spend for any given priority, Chen said.

All but a handful of the previous five dozen emergency declarations over the past four decades have imposed trade restrictions or other economic sanctions against nations for their aggressive behavior and human rights abuses.

A May 22, 1997, declaration by Bill Clinton prohibited new investment in Burma. An April 3, 2014, declaration by Barack Obama imposed sanctions on leaders of South Sudan. The very first emergency which is in place to this day was declared by Jimmy Carter to impose sanctions on Iran after the 1979 revolution.

Only a handful of the declarations have dealt with national policy outside of targeted sanctions. One, declared by Ronald Reagan in 1983, reinstated a ban on the export of technologies that could be used for military purposes. Congress had allowed the ban to lapse but did not oppose his use of the Emergencies Act to put it back into force.

Another came in 2005, when George W. Bush, without technically declaring an emergency, decreed the suspension of a requirement to pay workers involved in the Hurricane Katrina recovery a certain minimum wage. Democrats and some Republicans did object to that one, and set a vote in the House to disapprove it . Bush rescinded his decree the day before the scheduled vote.

Not one of the previous emergencies, however, did anything close to what Trump is trying to do: pay for a project not only without Congress approval, but after Congress has specifically rejected his request several times.

The law was intended to authorize a president to take action in a crisis that was unfolding too quickly for Congress to respond, Chen said. Congress has, in fact, made a decision. Congress has chosen not to give him the funds that he would like.

It is not clear if or when Trump would actually declare an emergency. Republican senators have already expressed their fears that enough of them would side with Democrats on the issue that a law rescinding the emergency would clear Congress, forcing Trump to back down or veto it a choice that would likely lead to an attempt to override the veto. White House officials would say only that Trump is considering all his options and that he would do everything necessary to deal with the crisis at the border.

Chen, though, said that Trumps actions threatening an emergency, but only if Congress refused to give him what he wanted have undermined the legal basis for one.

The longer he waits and flirts publicly with the concept of declaring an emergency, the more he weakens the case that there is an actual emergency, Chen said.