In Washington circles, the phrase "state of emergency" implies, well, an emergency — something that requires drastic government action.

But in a rambling statement last Friday, Mr Trump announced that he was declaring an emergency so he could get more money for his wall with Mexico than Congress had authorised.

Even as he announced the emergency, Mr Trump (inadvertently) called into question its urgency, saying: "I could do the wall over a longer period of time. I didn't need to do this, but I'd rather do it much faster."

He's seizing on a tactic that's been used 60 times since the power was granted following the Vietnam War in 1976. But with it came a safeguard: that Congress can challenge an emergency declaration.

In getting more money for his wall, Mr Trump is attempting to wrestle funding power away from Congress — and its triggered an inevitable confrontation two years in the making.

Instantly, the two most prominent Democrats in Washington — House Speaker Nancy Pelosi and Senate Democratic Leader Charles Schumer — warned Mr Trump they would act.

"The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available," the leaders said.

Sorry, this video has expired Donald Trump declares a national emergency at the US-Mexico border.

How will Congress fight back?

Once Congress returns on Tuesday, politicians could begin a series of actions.

One is to pass a resolution declaring that no such emergency exists.

Both the House and the Senate would need to approve it by a simple majority.

That's pretty much guaranteed in the Democratic-controlled House.

In the Senate, Republicans have a slim majority, so the Democrats need some Republicans to defect to their side.

That is not impossible, since Mr Trump's manoeuvre has offended some of his party members, including Maine senator Susan Collins, who called it "a mistake" and Kentucky senator Rand Paul, who tweeted that "extraconstitutional actions are wrong, no matter who does them".

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There isn't believed to be a situation where either house of Congress has voted to negate a state of emergency.

In 2005, George W Bush declared a state of emergency in the wake of Hurricane Katrina that would have suspended wage laws during rebuilding of the area.

A Democratic lawmaker submitted a resolution to negate the emergency and a vote was scheduled, but Mr Bush withdrew the declaration before the vote took place.

Should the resolution pass this time, it would go to Mr Trump, who has already forecast that he would veto it — which would be the first veto of his term.

Congress can override a veto if Democrats can get two-thirds of each house to go along.

But that's not likely, unless Republicans decide en masse to desert the President.

Then come the lawsuits

But there are more risks to Mr Trump's emergency. Congressional leaders could sue the President, something he predicted on Friday.

"And then we'll end up in the Supreme Court, and hopefully, we'll get a fair shake, and we'll win in the Supreme Court, just like the [Muslim travel] ban," Mr Trump said.

However, there are so many others standing in line to sue the administration that Mr Trump's challenge will not only be from Congress.

Leading that group are the owners of land that the United States would seize in order to build the border wall.

Under US law, the government can take land for a public use under the principle of "eminent domain", but property holders are unlikely to let go of their land without their own court challenge.

The Government is still dealing with lawsuits filed in 2006, when Mr Bush approved an original series of fences along the Mexican border.

Looming behind them is the state of California, which stands to be most affected by any new structure built on the Mexican border.

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On Friday, the state's governor, Gavin Newsom, and its attorney-general, Xavier Becerra, accused Mr Trump of creating a crisis and declaring a "made-up national emergency".

They may be joined by the state of Nevada, if the wall construction takes federal funds that were slated to go to projects there.

Even as the prospective legal challenges pile up, it's crucial to remember that Mr Trump's entire professional life has been rife with lawsuits.

He has frequently threatened to sue all manner of people and organisations, from journalists and their news outlets, to the Culinary Workers and Bartenders Union, to Republican presidential contenders John Kasich and Ted Cruz, to the co-author of his best-selling autobiography.

Mr Trump even ranted on Sunday against Saturday Night Live, the satirical program that he has hosted twice, when it mocked him over the emergency declaration.

This coming Sunday brings the Academy Awards, but it may be hard for the Oscars to match the drama underway in Washington.

Micheline Maynard is a journalist and author who writes about American politics.