President Donald Trump has weighed in on executive power, saying he could declare a state of emergency to build his wall, even though he may be bitterly opposed by the new Congress. "I can do it if I want," he told reporters last week, adding that the White House can "call a national emergency because of the security of our country."

It's possible that the president, who is addressing the nation in prime time on Tuesday night, is right. But probably not.

The Constitution does little to define national security powers and how the branches of government share them. Article I gives Congress the power to support armies and declare war, and call upon the militia "to … suppress insurrections and repel Invasions." Congress also has the power to regulate international commerce and establish the rules of naturalization.

Article II vests the president with the oft hazy "executive power," the power to make treaties, and it names him commander in chief of the armed forces. Left vague is the president's command over foreign affairs and the relative authority between the two branches in this area. Also vague is the president's enumerated power during emergencies, other than suspending "Habeas Corpus."

Accordingly, most of the president's emergency power derives from statute — laws enacted by the same Congress that might now oppose his actions. The Brennan Center for Justice has identified 136 statutory powers available to the president upon declaration of a national emergency.

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So, then, the source and extent of the president's emergency power is complicated. His authority to act contrary to the wishes of Congress is also complicated. It's happened before, and the Supreme Court has weighed in.

In the middle of the Korean War, in 1952, facing a threatened strike by steelworkers and the loss of critical war production, President Harry Truman invoked his emergency powers and directed his secretary of commerce "to take possession of the steel mills, and to keep them operating."

The steel mill owners immediately filed suit, and, in record time, the case arrived at the Supreme Court, which struck down the executive order. The six justices in the majority each wrote separate opinions, meaning they arrived at this conclusion for different reasons. In this landmark case, Youngstown Sheet & Tube Co. v. Sawyer (Truman's secretary of commerce), the assertion of emergency power ultimately failed because it lacked authorization "from an act of Congress or from the Constitution itself."

Nationalizing the steel mills is a markedly different executive action than building slat fences along the border, and 1952 was a different time than 2018. But Justice Robert Jackson's famous concurring opinion in Youngstown attempts to give some future guidance to the courts in evaluating the executive's power during a national emergency.

Jackson divided unilateral presidential actions, like the one proposed by Trump, into three categories. First, when the executive "acts pursuant to an express or implied authorization of Congress," then there's little controversy because he speaks for the entire federal government. What exactly constitutes an "implied" authorization by the legislature is open to interpretation, however.

The second category is the "Twilight Zone." When the president takes action, but Congress has neither granted nor denied his authority, and there is a "zone of twilight" in which the executive and legislative branches have shared authority. In these cases, the president might act where Congress has remained silent, but that also raises the question of how to define Congress' "silence" on a subject.

Trump's proposed action falls into the third Youngstown category: When the president "takes measures incompatible with the expressed or implied will of Congress," his power is at its "lowest ebb." Here, the Supreme Court would have to deny any congressional power to act in the controversy to sustain the president's exclusive control. In this category, when the president acts in defiance of Congress, he should usually lose — but not always. Indeed, in a 2015 case, the Court upheld the rare exercise of presidential power in category three.

If Trump invokes emergency powers against the express will of Congress, he acts at his "lowest ebb" of power. The action would be immediately challenged, and it would likely arrive at the Supreme Court in record time, as Truman's steel seizure did. Under Youngstown and the cases that have followed, the odds are against the president. But he still could win.

Danny Cevallos is an MSNBC legal analyst. Follow @CevallosLaw on Twitter.