President Donald Trump has declared a national emergency to access funding to build a US-Mexico border wall. But it's not as simple as just calling a national emergency today, and starting construction on the wall tomorrow.

For a start, Democrats and other groups have announced they will try and block the emergency declaration, leading to a battle in the courts.

Here's how it could play out.

Is it normal for presidents to declare a national emergency?

Yep. They do it all the time.

In fact, with this latest declaration, there are currently 32 active "national emergencies" underway in the US — some of which have been in place since Bill Clinton's presidency.

Before today Mr Trump himself had already issued three during his presidency, ranging from global human rights abuses to election interference.

It's been 40 years since there wasn't an active national emergency.

Thanks to a 1974 law, the president has the special power to declare such a state at any time.

Calling something a "national emergency" means the president can access funds to fix whatever issue has been deemed as the emergency, bypassing some of the checks and balances available to Congress.

Things like:

War

War Natural disasters

Natural disasters Epidemics

Most national emergencies are non-controversial and receive bipartisan support.

But Congress can veto a declared emergency at any time, with a majority vote in both the House and the Senate. More on that below.

The test will be if there is an actual emergency or not

Legal scholars agreed that a court test would likely focus on whether an emergency actually exists on the US southern border.

The limits of presidential power over taxpayer funds is also likely to be tested.

Mr Trump triggered a partial shutdown of the Government last year by demanding the inclusion of $US5 billion for his proposed wall in any legislation to fully reopen agencies.

But his declaration of a national emergency is likely to result in a long court fight, possibly stretching into Mr Trump's 2020 re-election bid and emboldening critics who accuse him of authoritarian tendencies.

Mr Trump is expected to argue illegal immigration constitutes a national emergency, entitling him to mobilise the military to the border and use Defence Department personnel to design and construct a wall.

One way to challenge such an assertion would be to demand Mr Trump show in court that an emergency actually exists.

The courts and Congress can step in here

A 2007 report by the Congressional Research Service, a nonpartisan research arm of the legislature, said:

"Both the judiciary and Congress, as co-equal branches, can restrain the executive regarding emergency powers."

The National Emergencies Act of 1976 was meant to create a congressional check on presidential emergencies. Under the law, the president must notify Congress and the public about an emergency declaration.

Congress can override Mr Trump's declaration, but it requires approval by both chambers.

As of January, the House of Representatives is under the control of Democrats.

Sorry, this video has expired With Nancy Pelosi in charge of the House, Mr Trump is likely to run into problems.

They've already said they would be opposed to using a national emergency to build the wall.

Given the Republican majority in the Senate, it's unlikely that a veto will occur.

Instead, Congress will fight the action through the courts, which promises to be a long and arduous process.

Mr Trump might still run into other problems

Things are about to get tricky for him.

Mr Trump will need to get his wall money out of funds already allocated by Congress for other purposes.

He could run into problems if he tried to shift funds dedicated to something else over to his wall.

Congress does give federal agencies some money without clear priorities. Mr Trump would likely need to tap that kind of funding to avoid violating congressional authority.

Mr Trump has not shied away in the past from unilateral action.

But the liberal Centre for American Progress said a national emergency strategy would be "illegal, plain and simple".

ABC/wires