If the English Parliament had accepted the pardon, the King would have been free to screen ministers from parliamentary inquiry and impeachment — and thus put himself beyond the reach of Parliament. For members of the House of Commons who viewed the impeachment power as a means to preserve government and bring corrupt ministers to heel, the act of executive clemency could not be tolerated.
The framers of the Constitution vested in the US President less, not more, power than that attributed to the King. But let’s say that Trump, having flouted the norms so many times before, does attempt to pardon himself for any crimes he might have committed outside the purview of the ban on impeachment-related pardons.
And when Trump effectively tried to assert absolute power in Washington on January 6 by inciting his followers to overturn an election, he triggered the unpardonable impeachment charges that should — if all else fails — be a slam-dunk in a court of law.