U.S. President Donald Trump says he “absolutely” has the authority to declare a national emergency as a way of achieving his promise to build a wall along the border with Mexico. That’s one way he could try to end the budget standoff that has led to a lengthy government shutdown without abandoning a project he made the centerpiece of his campaign. Opposition Democrats, who have refused to authorize the billions needed for the wall, say he’s wrong.

1. What exactly is a national emergency?

It’s a declaration by the president that gives him special, temporary power to deal with a crisis. In the past, most such invocations have been related to foreign policy, like prosecuting a war or responding to a global trade threat. International concerns explain most of the 28 currently active national emergencies. On a few occasions, however, presidents have also used emergency declarations to further their domestic policy goals.

2. How would an emergency declaration help Trump?

In theory, it would let him redirect federal money allocated for other purposes and use it to at least get construction started on the wall he wants to build on the border with Mexico. Under the law governing the Pentagon, for instance, if the president declares a national emergency, the defense secretary is allowed to redirect money from military construction funds for projects “necessary” to support the deployment of U.S. armed forces. Or the defense secretary could terminate or defer the construction of Army civil works projects and apply those funds to “authorized civil works, military construction, and civil defense projects that are essential to the national defense.” Another option being discussed: shifting funds from a disaster-spending bill passed by Congress that includes $13.9 billion in not-yet-spent money.

3. Would that free up enough money to build the wall?

The president’s request for $5.7 billion in wall funding represents more than half of the $10.3 billion appropriated for military construction this year and would mean sidelining politically popular projects. That could particularly hit military families, with the government slated to spend $1.6 billion of that budget on family housing, $352 million for medical facilities, and $267 million on educational facilities.

4. Does this really qualify as an emergency?

There’s a case to be made that that’s entirely up to the president. Emergencies have been declared during crises large (the Civil War, the Great Depression, the aftermath of the Sept. 11 terror attacks) and small (remember the 1970 postal strike?). In 1976, Congress adopted a law giving the president broad authority to declare a national emergency. But the law, passed as part of a sweeping set of legislation designed to restrain presidential powers after the Watergate scandal, also demands that the president invoke specific statutory authority for emergency actions.

5. Would an emergency declaration be challenged?

Count on it. Congressional Democrats have already said they would mount a court challenge to any use of emergency authority. “The president’s authority in this area is intended for wars and genuine national emergencies,” Evan Hollander, spokesman for the House Appropriations Committee, said in a statement. The Congressional Research Service, in a Jan. 10 report, said invoking a national emergency for this purpose “would raise a variety of novel legal issues,” including whether Trump is addressing a problem that really “requires use of the armed forces” and whether the planned wall would really “support such use of the armed forces.”

6. What’s happened in previous fights?

Courts have limited the president’s emergency powers before, including in a 1952 Supreme Court decision in which the high court said President Harry Truman could not use an emergency declaration to seize steel factories to ensure production during the Korean War. But since laws governing emergency authority have changed, some experts say it would be difficult to predict how the case would be viewed by the current Supreme Court in which Republican-appointed justices hold the majority.