1/ The now-infamous Watergate-era OLC (DOJ) memo was bolstered in 2000 when it was repeated, cited and effectively adopted by another OLC memo in that year. The upshot: it says you can't indict a sitting president because doing so might distract him/her from a president's duties.
2/ The problem is that SCOTUS has ruled you *can* distract (as it were) a president for many types of legal proceedings. Nixon was ordered to produce physical evidence; Clinton lost unanimously when he tried to avoid having to give testimonial evidence himself. But there's more.
3/ Trump's handpicked Attorney General, Bill Barr, recently said in a TV interview thatphysical/testimonial evidence asidethe DOJ can *and should* issue *public charging decisions* when they believe a president may have committed a crime. He chastised Mueller for not doing so.
4/ So the "don't distract the POTUS" narrative seems faded: you can make a POTUS produce physical evidence, make a POTUS testify, publicly state an intent to charge... the only question is if you can *try* him/her. So why Mueller didn't even subpoena him for testimony is unclear.
More: https://threader.app/thread/1153354684293406721
Laurence Tribe has also weighed in on this issue
https://www.lawfareblog.com/yes-constitution-allows-indictment-president