Give President Donald Trump credit for being transparent about his plan to exert executive authority in what many view as a challenge to Congress. At least Trump, unlike his predecessor, is not engaging in a covert operation.

For months, the president has insisted at least $5.7 billion is needed to erect new barriers against illegal immigration across the nation’s southern border. Last week, in a compromise intended to avoid another partial shutdown of federal agencies, Congress appropriated about $1.4 billion for the purpose. Not enough, Trump declared.

On Friday, he told reporters at the White House he is declaring a national emergency over illegal immigration. He believes that gives him the authority to take money already appropriated for other initiatives and transfer it to the border barrier. About $8 billion will be shifted out of other accounts to expand the line of border barriers.

At first glance, the obvious question many taxpayers may have is why, if that much money is not needed for the purposes originally earmarked, it was in the federal budget in the first place. Too bad members of Congress never seem to ask it. But some, including a few Republicans, are asking whether Trump’s action is constitutional. That will be tested in the courts.

Trump is not the first president to divert federal funding. Former President Barack Obama did it, illegally funneling billions of dollars into the Affordable Care Act. Some of it went to private insurance companies in subsidies intended to keep Obamacare insurance premiums artificially low. Obama did all that in secret, without even the pretext of a national emergency. His action was purely political.

So Trump earns a pat on the back for openness. What about constitutionality, however? Is he infringing upon Congress’ authority to appropriate money? Lawmakers are right to be concerned. If Trump can declare an emergency on illegal immigration, what is to prevent a future president from doing so for some other pet project?