What happens if Trump declares a national emergency to build a wall?

(CNN) - Just days before the deadline to avert a second government shutdown, border talks among congressional negotiations appear to be making little headway.

President Donald Trump has hinted he could bypass Congress and get the funding for his proposed border wall by declaring a national emergency - and the White House has drafted documents to do it.

But what does that mean, and when would it take effect?

Believe it or not, the country is actually already under 31 states of declared emergency, according to a review of documents from the Congressional Research Service and the Federal Register.

One goes all the way back to the Carter administration, a response to the Iran hostage crisis in 1979.

Trump himself has already made three, including one to punish foreign actors found to interfere in U.S. elections.

Typically, they're not controversial because there's broad consensus about the seriousness of the issue - like President Bush's emergency declaration following the 9/11 terrorist attacks.

Congress can, technically, vote to overturn a declaration. But Senate Majority Leader Mitch McConnell wouldn't say whether his Republican-controlled chamber would do that.

“Well, we don't know what route the president's going to take so I'm not going to speculate on it at this point,” he has said.

An initial congressional override simply requires a majority vote. But if the president were to veto Congress, they'd need two-thirds of each chamber to override it.

The judicial branch may also have a say, if - as expected - legal challenges questioned whether the president is overstepping his authority, and whether what's happening at the border reaches the threshold to warrant such a move.

If the president does make the move, it’s not clear what immediate effect the declaration would have on actual wall construction – particularly regarding practical matters such as fielding contracting bids from construction companies, directing military supervision or other undetermined factors.

According to The Washington Post, the administration could target as much as $10 billion provisioned for “military construction projects” under the National Emergencies Act.

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