Dean was indicted for several counts of obstruction of justice for his role in a specific CRIME that had been committed and for which there was no dispute that it was a crime. He was not subject to subpoena before Congress until he had been indicted and convicted and/or pled guilty. There was no doubt about it that people broke into the Democratic National Committee headquarters located in the Watergate Hotel, and strong evidence that the Administration knew about it. John Dean III was identified by the FBI as the master mind behind the cover up and he was indicted--INDICTED--by the FBI on numerous counts of obstruction of justice mainly paying hush money to keep people quiet.
He plea bargained to turn state's evidence in return for him pleading guilty to one count of obstruction of justice and thereby was an important component to the indictment and imprisonment of others on the White House team. He was sentenced to four years in federal prison, but his cooperation with the prosecution was a factor in him serving only four months of that sentence. He was disbarred however and it ended his legal career.
Up to that point there was nothing to tie President Nixon to the Watergate scandal. Even "Deep Throat" couldn't do that. It was John Dean's testimony re the White House tapes and the fact that John Dean had ordered hush money and John Mitchell, Nixon's A.G., had illegally used campaign funds to pay operatives to spy on the Democratic Party which of course was also a crime. That gave the Supreme Court enough ammo to order the White House to release the tapes and the rest, as we say, is history.
Now compare that with the current situation. No crime has been identified by any legal authority related to President Trump or the Executive Branch. ALL indictments related to the Mueller investigation involving people who at one time were on on the President's campaign thus far have been for crimes totally unrelated to the President or the campaign. Or their indictment/guilty plea was due to having been caught making some kind of error or omission of fact and thereby falling into a perjury trap as a result of their testimony to Mueller and/or his team members. IMO and according to several legal minds, all that latter group would NEVER have been charged with anything if Mueller hadn't hoped to bully, threaten, frighten, extort them into divulging some dirt on the President. As it was their names and reputations were dragged through the mud, their friends and family were threatened, and most were bankrupted in the process. It was a nasty, dirty, indefensible, and unacceptable thing to do to any Americans.
And now the hate masters in Congress are hoping beyond hope to find a crime, ANY crime, to pin on the President in advance of the 2020 election and probably get rid of Barr and his special counsel before they expose any malfeasance and misconduct among their own in the FBI and Obama justice department, or Hillary, or possibly Obama himself. At the very least they intend to keep the negative press going and of course the MSM is cooperating with that as are those on social media, message boards and the such who dutifully parrot the assigned talking points.
So far no identifiable or prosecutable crime has presented itself and time is running out for them as the I.G. and the current Justice Dept. closes in on what could be some very damning news for the Democrats, their surrogate media, and their whole I-hate-Trump scene.
McGahn and nobody else on the White House staff, past or present, should be available to them for a fishing expedition in their attempt to complete a coup of a lawfully elected and inaugurated U.S. President they hate.