According to an April 19th letter from the White House legal counsel Emmet Flood, to Attorney General William Barr, Donald Trump has the right to instruct his advisers to not testify before congressional probes related to the Russia investigation.
“It is one thing for a president to encourage complete cooperation and transparency in a criminal investigation conducted largely within the Executive Branch. It is something else entirely to allow his advisers to appear before Congress…” the letter said.
Every since the Mueller report was released there has been tension between the White House and congressional Democrats. Jerrold Nadler, the chairman of the House of Representatives Judiciary Committee, has already threatened to hold William Barr in contempt for not releasing a full, unredacted report.