If you took a poll at a Donald Trump rally of his single most important promise, that wall would be it.

Why? Because it's a concept everyone can understand.

Walls are for keeping things out — in this case, Mexicans.

And as far as Mr Trump is concerned, far too many of them are bad.

Build a wall, and drugs and crime will be kept on the southern side of that beautiful barrier. Simple.

Sorry, this video has expired In March, Donald Trump told Congress the wall would go ahead

The problem is, Congress has refused to give Mr Trump the billions of dollars needed to build it.

Now he has repeated his threat to shut down the entire US Government if he can't get the money. It's a Mexican stand-off.

With the United States facing its biggest hurricane disaster since Katrina, you might have thought this was not a great time to continue the wall wars with Congress.

After all, the battered and flooded cities in Texas and Louisiana need all the federal funds they can get right now.

A makeshift barrier separates the US and Mexico in Tijuana ( AP: Julie Watson )

Yet still, the President sees this as an opportunity to fulfil his promise and brick up a relationship with Mexico at the same time.

And while Mr Trump concedes the American taxpayers will initially come up with the billions for the border barrier, he still says eventually the Mexicans will pay, one way or the other.

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If that isn't all enough for the Mexican Government to deal with, the President has repeated the North American Free Trade Agreement (NAFTA) with Mexico and Canada will be renegotiated or terminated.

Mr Trump accused the Mexicans of being difficult, trying to hang on to what the President says is a dud deal which has stolen US jobs and sent them south.

The US President is playing to his supporters.

He is demonstrating that while he may have backflipped on issues like withdrawing from Afghanistan, his most repeated promise will be kept.

And he's willing to withhold his signature on debt extensions that will effectively shut down the Government.

Mr Trump's supporters will believe him when he tells them it's all the fault of the Washington insiders, including those in his own party.

It's the alligators in the Washington swamp tearing up reason, denying protection to honest Americans from Mexican criminals and their drugs.

Paul Ryan, the top Republican in Congress, says the US Government does not have to choose between border security and continuing operations. ( Reuters: Joshua Roberts )

The US President is not letting Hurricane Harvey deter him from squeezing Congress to the point of a possible shutdown.

He will shortly tour hurricane-affected Texas.

The Mexican Government has offered to help with the rescue and recovery operation, but it's not an offer Mr Trump is likely to accept.

He has other plans for setting down a new relationship, drawn in the dry border dirt and marked by brick and wire.

It's his line in the sand; physical, literal and easily understood.

And he'll go to war with Washington to get it done.

No mere hurricane is about to slow him down.