Numerous top aides have testified before Congress.

“Dan Bartlett, counselor to Mr. Bush, has said it is ‘highly unlikely’ that the president would waive executive privilege to allow his top aides to testify publicly. One Republican strategist close to the White House…said: ‘No president is going to let their senior staff assistant to the president go testify. Forget that. They might agree to do an informal interview, but they’ll never testify.’ … Democrats, citing a report by the nonpartisan Congressional Research Service, say presidential advisers, including 47 from the Clinton administration alone, have frequently testified before Congressional committees, both while serving the president and after they had left the White House.”