Congress wanted to subpoena the former White House Counsel, Don McGahn, to question him about whether President Trump obstructed justice during the Mueller investigation. The White House refused to let McGahn testify and Congress sued to make him.
President Trump argues the courts cannot intervene in a dispute between Congress and the president. But this is the opposite of what he argued in Mazars and Deutsche Bank. In those cases, Trump asked the courts to protect him from a congressional investigation that was about to obtain his tax returns. If the president is allowed to have it both ways, then neither Congress nor the courts will be able to check presidential abuses of power. The president would be above the law.