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After his failure to win support from Congress for his demand to fund the building of his border wall, Donald Trump was left with a series of unpalatable choices.

Admit total failure on your key campaign pledge. Or go nuclear.

He's gone nuclear.

By declaring a state of emergency he will be able to raid other departmental budgets to cobble together $8bn for construction on the southern border.

He will show his base that he is true to his word.

He will argue he is fighting their fight, to staunch the flow of illegal immigrants and dangerous drugs into the country.

And it is undoubtedly true that a lot of people from Central America are trying to enter the US illegally - even though less than in previous years.

And a lot of drugs, too, are flooding into the US, courtesy of the Mexican drug lords.

There is a separate debate about how effective the blunt instrument of a wall would be.

Some argue that more effective would be the use of technology and reinforcing the numbers of border patrol officers.

But as I say, let's leave that to one side. The trouble with going nuclear, is there is fall-out.

This has been presented as a predictably partisan issue.

On one side of the wall, Republicans; on the other side, Democrats.

But by going nuclear the president has made it more complicated than that. There are a lot of Republicans - in the Senate and in the House - deeply uneasy about what Mr Trump is doing.

Why? Because the constitutional arrangement of the US is that Congress controls the purse strings and allocates funds. Not the president.

This is a major land grab by the president.

It undermines the powers of Congress and sets a very dangerous precedent.

Let's spin forward a few years, and it is a Democrat who is in the White House.

There is a mass shooting somewhere. The president can't force through much tighter gun control measures through Congress, but will now have the Trump card to play.

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I see your objections, and raise you a national emergency.

Bang (wrong word). Tighter gun control measures are introduced.

On healthcare, ditto. And what about climate control? Yep that too. Lawmakers could be totally by-passed.

The emergency powers were designed for a genuine national emergency.

If the situation on the border is a genuine national emergency, why has it taken the president over two years to make this move?

You can be sure that the Democrats will be considering a legal challenge that will wind its way up to the Supreme Court. And that will delay any building work.

It is likely that over the coming months, the lawyers in Washington will be far busier than the bricklayers in Arizona and Texas and California.

And the legal challenge will contain one central question - is this a national emergency, or a political emergency?

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