But if Trump issues a national emergency declaration to build a wall on the U.S. border with Mexico, as Senate Majority Leader Mitch McConnell (R-Ky.) suggested he would on Thursday, Trump would be using that power in a way that it has never really been used before. He would also be taking an action that polls have repeatedly shown runs against the will of the American public.

According to the Brennan Center for Justice, there have been 58 national emergency declarations since the law allowing such declarations was passed in 1976. Of those, 31 are still active — eight of them having been enacted in the prior century.

The vast majority of these declarations focus on international disputes, including the implementation of sanctions. Several others focus on combating weapons proliferation or export controls, as this Washington Post graphic outlines. There have been a few declarations meant to address specific crises, like a swine flu outbreak in 2009 or the Sept. 11, 2001, terrorist attacks.

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Trump’s wall declaration would be one of those outliers. It would also be the only one specifically focused on trying to launch a construction project.

A presentation from the Congressional Research Service that was published last week notes that there have been instances in the recent past in which construction was authorized under the auspices of the National Emergencies Act to leverage military funding, as Trump has hinted he intends to do. In 1990, Executive Order 12734 invoked the NEA for Operation Desert Shield in the lead-up to the Gulf War. In 2001, Executive Order 13235 did the same in the aftermath of the attacks on the World Trade Center and Pentagon.

This gets to one of the central critiques of using the National Emergencies Act to build the wall: Does an increase in immigrants coming to the border actually constitute an emergency that’s equivalent to a flu outbreak or a massive terrorist attack? It’s been noted repeatedly that apprehensions of unauthorized immigrants at the border, which have increased in recent years, still pale next to the number of apprehensions recorded even 20 years ago. What’s more, Trump’s arguments about the danger posed by not building his wall have been repeatedly shown as misleading or false.

That CRS report made another important point: Much of the land at the border isn’t under the control of the federal government (much less the Department of Defense).

Whether Trump can declare an emergency to build his wall is one thing. (As is the almost certain legal and legislative backlash such a move would prompt from congressional Democrats.) Notice the light yellow on the map above. It’s all land that’s not held by the federal government. There would need to be a massive deployment of eminent domain to obtain land for the wall’s construction, which would likely not sit well with many Americans.

Polling has repeatedly shown that even those Americans whose land wouldn’t be seized oppose Trump taking this action.

In January, two-thirds of Americans — including more than a quarter of Trump’s own party — expressed opposition to a national emergency declaration in Quinnipiac University polling.

A CNN poll earlier this month had a similar result. A Fox News poll released Wednesday showed slightly more support for a national emergency declaration, but nearly 1 in 5 of those who voted for Trump in 2016 opposed his taking this action. Overall, support for the move in the Fox poll was similar to a poll the network conducted in January.

A Post-ABC News poll released in January makes another point clear: Opposition to using a national emergency to build the wall is actually higher than opposition to the wall in general.

In other words, as unpopular as building a wall on the border is (and a majority of Americans consistently indicate that they oppose its construction), a decision by Trump to step around Congress is less popular still.

Not with Trump’s supporters, though. And not with Trump, who has frequently looked to remove the shackles of constitutional checks and balances. He seems to have found a way to do so — until Congress and the courts offer a response.