Before, during, or after he serves.

One primary idea behind impeachment imo is to remove a president from office so he can be prosecuted if he broke the law. In other words, Nixon broke the law while in office, saw the writing on the wall that he’d be impeached and removed, and then it took a pardon by Ford to prevent prosecution. Not some automatic immunity because he was president while being a scumbag. Nixon admitted his guilt by accepting the pardon.

We don’t have a lot of precedent for lawless presidents, but I think they confirm that a pres doesn’t become dictator with no criminal constraints just by winning elections. It’s another giant pile of Trumpistani shit attempting to stain the constitution. Which has become redundant by this point.