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As the White House descends further into chaos, no one can discern President Donald Trump’s next move until he makes it.

But Trump and administration aides have signaled that he views a presidential declaration of emergency as an exit ramp from the standoff with Congress that has shut down a large chunk of the government for 18 days now. It would, simultaneously, let him assent to reopening federal agencies and assert unilateral authority to order construction of a wall along the U.S.-Mexico border.

Bracing for Trump’s decision, congressional aides in both parties say the potential scope of presidential action ranges from very big to very small.

A president’s authority to declare a national emergency gives him the power to redirect taxpayer funds Congress has already approved. In conversations with lawmakers, the White House has pointed toward two different pots of Pentagon money. One, containing less than $1 billion, is for curtailing drug shipments. The other, containing $22 billion, is for military construction projects.

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The military construction pot might even be enough to pay for whatever barrier Trump wants across the 2,000-mile border, whether constructed of concrete or steel or something else. If Trump goes big, he could announce his intention to do exactly that.

Yet the bigger he goes, the bigger the backlash he risks.

An emergency declaration related to border control would face immediate legal challenges, just as his attempt to ban travel into the United States from some majority-Muslim countries did. The more miles of wall Trump attempts to build under an emergency declaration, the more potential challengers — Congress, private landowners, affected local governments.

Moreover, a recent 17-page report to Congress shows that the pot of military construction money contains funds previously designated for projects across the country. That would ensure resistance from both red and blue states eager for those projects. And at a time when Trump speaks openly about the prospect of impeachment proceedings, the weak factual basis for a border emergency claim would invite House Democrats to assert an unconstitutional abuse of executive power.

But a little-noticed administration maneuver last fall points to a far more limited possibility.

It could be completed by only touching the small pot of anti-drug money. It wouldn’t require diverting cash from military construction projects elsewhere. It would only affect 31 miles of the Mexican border on a federal explosives-testing site in Arizona.

Last fall, the Pentagon informed Congress that the Navy had committed $7.5 million to “advance planning and survey efforts” for new and improved border barriers on the Barry M. Goldwater Range. Calling the range “a known drug-smuggling corridor,” Assistant Defense Secretary Kenneth Rapuano wrote that such barriers “will both protect BMGR from such illegal activity and address human life and safety concerns by deterring unlawful entry onto an active bombing range.”

In an October letter to Democratic Rep. Adam Smith of Washington, who now chairs the House Armed Services Committee, Rapuano said no construction had occurred on the range. But he noted that the administration was “reviewing its authority and funding options.”

Expressing strong opposition, Smith and his Democratic colleagues estimated that the 31 miles’ worth of barrier along the range would cost $450 million. The anti-drug account that the Pentagon might try to tap contains $760 million, according to a Republican congressional aide.

Trump has the opportunity to clarify his intentions in a televised prime-time address on the issue Tuesday. If recent form holds, lawmakers will find out at the same time everyone else does.