Brian Snyder / Reuters

Updated at 11:19 a.m. ET on January 14, 2019.

President Donald Trump tonight will hold a nationally televised speech to address the impasse over the border wall. Because Democrats have refused to appropriate approximately $5 billion for the construction project, Trump is reportedly mulling alternative avenues to deliver on his campaign promise.

Here’s what he can do under the law, and what he can’t.

Trump has two traditional statutory avenues.

He could ask Congress to pass a new piece of legislation specifically for the construction of a contiguous physical wall. But the Democrat-controlled House would never pass such a bill, and Senate Democrats could filibuster the legislation if it did.

To get around this blockade, Trump could cite his authority under the Secure Fence Act. The 2006 act, which garnered bipartisan support during the George W. Bush administration, allocated funding to construct “fencing” and “physical barriers” along a 700-mile stretch of mostly federally owned land at the border. (The Southwest border spans roughly 2,000 miles.) The law also permitted the secretary of Homeland Security to secure the border through actions that are “necessary and appropriate” for security purposes. But the existence of this provision gets Trump only so far; he still needs Democrats in Congress to release funding for additional fencing, and they’re not going to do that.

Trump is now reportedly weighing the option of invoking his powers under the National Emergencies Act of 1976 to declare a national emergency, send the military to the border, seize land, and construct the wall using funds from the military budget. Trump has consistently stated, with little support, that illegal immigration across the U.S.-Mexico border has turned into a legitimate “national emergency.”

Without question, Trump may invoke the 1976 act and declare a national emergency. Congress, however, may terminate the emergency by resolution. If Congress succeeds in passing such a resolution over a possible presidential veto, then the “national emergency” would be suspended, and any ongoing activity by the military along the border would be halted.

What if Congress doesn’t pass a resolution?

Some have argued that, even under his emergency powers, Trump simply cannot require troops to build a border wall. Indeed, existing statutes prohibit the Army and Air Force from executing domestic law unless authorized by the Constitution or Congress.  But the law is not so clear-cut. For example, 10 U.S.C. 2808 authorizes the reallocation of certain military funds in an emergency, while 33 U.S.C. 2293 permits a similar reallocation for emergency circumstances involving civil works. (The latter section, however, only permits the military to participate in “authorized” works, such as civil-defense projects. And it would be difficult to argue that Congress has “authorized” a border wall beyond the 700 miles of fencing.)

Trump’s wall could also founder on private-property rights. One-third of the land along the border is owned by the federal government, but the rest is owned by individuals, American Indian tribes, and state and local governments.

Last Friday, Trump said he would use a “military version” of eminent domain to acquire the land he needs. It’s true that Congress has delegated the power of eminent domain to executive agencies, including the Army Corps of Engineers. But the power to condemn land for a particular federal project must still be granted by Congress. The president cannot simply order the military to seize any land he wants—not even if he declares a state of emergency. Indeed, a plain reading of the National Emergencies Act does not expressly permit the military to seize private property at the president’s command without additional statutory authorization from Congress.

It seems unlikely, then, that Trump will get his border wall unless Congress has a change of heart. The courts, however, may see things otherwise.

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