President Trump doesn't want former White House Counsel Don McGahn in front of Congress.
The West Winger turned fascinating Mueller report witness was subpoenaed by the House Judiciary Committee for testimony last week, though Trump has publicly said he wouldn't let McGahn attend. McGahn isn't in the administration anymore, but that didn't stop the Justice Department from issuing a 15-page letter spelling out why McGahn could not legally be called to testify, and it didn't stop Trump from publicly directing him not to testify on Monday.
In the Monday letter to current White House Counsel Pat Cipollone, Assistant Attorney General Steven Engel wrote that "Congress cannot constitutionally compel the president's senior advisers to testify about their official duties." He specifically cited an opinion from former President Bill Clinton's Attorney General Janet Reno, along with precedents from throughout presidential administrations. White House Press Secretary Sarah Huckabee Sanders summed up this "bipartisan and constitutional precedent" in a statement, and affirmed that Trump had directed McGahn not to testify.
McGahn still hasn't said if he'll comply with Trump's order, but he faces a pitfall either way. If McGahn ignores Trump and testifies, he'll likely "damage his own career in Republican politics, but also put his law firm" at risk because Trump could tell its GOP clients to "withhold their business," The New York Times explains. But if McGahn steps out on Congress, he could face a contempt charge. Judging by Attorney General William Barr's jokes in the past few days, though, that doesn't seem to matter much.