Sen. John Hoeven, R-N.D., expressed confidence to reporters earlier this week that the move was legal:

“It’s legal. We worked through this very diligently with [the Office of Management and Budget].”

Max Stier, the CEO of the Partnership for Public Service and a former attorney, said that Trump’s move is well-intentioned — but likely illegal. He also said it is unlikely to get challenged:

“As a policy matter, it’s ridiculous. Congress and the president have failed to get appropriations passed for DHS. It shouldn’t be federal employees that suffer those consequences. … Government shutdowns are a manufactured crisis and federal employees not getting paid is the worst part of that manufactured crisis.

“There is no workforce in this country that can be told they have to work without pay except for federal employees. And it’s wrong. But this is not the right solution for it.”

David Super, a Georgetown Law professor, argued that Trump needed express authority from Congress to spend this money on TSA — and that he doesn’t have that:

“Under the Appropriations Clause of the US Constitution, in Article I, Section 9, the president may not spend money without an appropriation from Congress. Under the ‘Purpose Act,’ 31 US Code section 1301, ‘Appropriations shall be applied only to the objects for which the appropriations were made except as otherwise provided by law.’ Therefore, President Trump needed express authority from some law to spend this money on the TSA officers’ salaries.

“I am aware of no law that provides that authority. If the White House believed it had such a law, I would have expected it to invoke that law as soon as the partial government shutdown began. I also would expect it to specify that law in its order, which it did not do.”