David Cay Johnston, the author of the 2016 book "The Making of Donald Trump," sounded the alarm on Monday about Trump's refusal to release his tax returns.
During a panel discussion on CNN, Johnston said that defying a congressional request for the president's taxes would be a blatant violation of the law.
"The law is clear," he said. "It says the Treasury secretary or IRS shall turn these over. Congress has been doing this for 80 years. There's never been a case where they didn't turn it over. It is not for public consumption, it's turned over to the chairman for the use of the committee in closed-door session."
He then argued that if Trump could get away with defying Congress's request, it would mean that he is completely invulnerable to basic oversight.
"This argument that [acting chief of staff] Mick Mulvaney put forward boils down to one thing," he said. "This is the first step to a Trump dictatorship by asserting the president is above the law."
Watch the video below.