I have learned a lot from watching Rachel Maddow, whom I absolutely adore. From her show, I have understood that
there is absolutely nothing in the constitution that prevents the indictment of a sitting President. Apparently, "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
That was from Article II, Section 4, of the American Constitution:
https://en.m.wikipedia.org/wiki/Arti...s_Constitution
Agnew was threatened with being indicted unless he resigned because they were afraid that after Nixon left - the rationale being that they didn't want Agnew to end up as the President (Agnew was a crook), hence the threat. So Agnew quit, and got a slight slap on the wrist...
Maddow explained the why and how of that situation clearly, as follows:
The key point is that "under Justice Department rules, that is a reference in fact to the standing internal Justice Department policy that says a sitting president of the United States can't be indicted. It's not a law that says a president can't be indicted. It's not written into Justice Department regulation. It's just a department policy. And it is a policy that derives from a very specific place.
...
I mean, what the Dixon memo said in 1973, what that memo said was you could indict a vice president but incidentally you couldn't indict a president. And the way that the history of it has been remembered since then is that that 1973 OLC memo was written specifically with the Richard Nixon Watergate problem in mind and it was a definitive look at whether a president can be indicted, and even in the context of Watergate they believed that Nixon really it was about Agnew and specifically trying to get to an outcome where the answer would be, yes, you can bring charges against Agnew.
http://www.msnbc.com/transcripts/rac...how/2019-02-21