President Trump has invoked executive privilege to stymie congressional investigators. Another president, Richard Nixon, did the same thing. It helped Nixon hold onto power – but only for a while.
The Constitution gives Congress the power over the executive branch, which it's free to flex.
As the special counsel's investigation of Trump turns into a partisan battle in Congress, here are four key issues to follow.
The president and attorney general can try to keep the findings of Mueller's investigation secret. They'll likely use both the secrecy of grand jury proceedings and executive privilege to do that.
Laws that limit presidential power won't enforce themselves – Congress must act.
Did the attorney general help create a false story on why Comey was fired? Sessions' testimony to Congress provides no answers.
The law says official presidential records must be preserved. How do tweets figure in – particularly when they're altered or deleted?