From Jonathan Bernstein:
During the Watergate investigation, the Justice Department made sure that the White House was updated on every development, which White House Counsel John Dean then used to prepare the false testimony of the campaign and White House staffers involved, and otherwise keep Justice from learning the truth. It was the revelation of this situation that eventually produced the first Watergate special prosecutor, and the universal demand that he be replaced by another independent prosecutor after Nixon had him removed.
The outcome of all of this was a set of rules and norms about the independence of Department of Justice prosecutions from presidential and White House involvement. A lot of Watergate-era reforms have been either useless or counterproductive, but everyone up through the Barack Obama presidency agreed to abide by them, and they became an important component of how the U.S. system produces the rule of law.
It’s those rules and norms that President Donald Trump is bulldozing when he demands prosecution of his political opponents – a demand he’s making when there is, remember, already an inspector general investigation.
That’s not even counting the recklessness involved in outing intelligence informants.