You need to read about the measures great past Presidents like Abraham Lincoln, amongst others, took. It's gonna be an eye-opening experience for you if your comment is in good faith.
That's what's unprecedented. It's not abuse of power or corruption that's unprecedented. It's serial, bald faced lies to a willfully, spitefully credulous audience. Not even Nixon or his defenders were so unscrupulous, although it's fair to say that in Nixon lies some precedence.
I used to think it quaint that the dominant principle behind the 6th Amendment prohibition against compelling testimony against oneself remains the so-called cruel trilemma-- https://en.wikipedia.org/wiki/Ex_officio_oat). But I realize now that, even without the metaphysical threat of damnation, a society must leave space for even the most corrupt and malicious individuals to avoid outright lying. Lies are the greatest poison to civil society by threatening the very notion of accountability and therefore of fairness or justice. Even when we know somebody is lying it's crucial that we grant them the fiction of plausible deniability, a quid pro quo for not violating the cardinal rule. In so far as the body politic permits and tolerates open lying, it destroys the last and perhaps only common bond between citizens--commitment to a shared truth, even if only nominally.
When a politician says that they can't remember, or even qualifies what would otherwise be a lie with "not to my recollection", that's actually a powerful gesture--submission to a critical civic virtue. When we get to a point when politicians, especially national leaders, don't bother with such gestures, and supporters don't even nominally demand them let alone exact some price, we're in a very bad place.
EDIT: Now that I think about it, there's solid evidence that Reagan openly lied when he denied any knowledge about Iran-Contra. But I think it's telling that we've convinced ourselves he was suffering from Alzheimers, forcing plausible deniability upon him. Even most Reagan detractors have reflexively internalized that narrative. (Obviously he was suffering from Alzheimers, but it's not very reasonable to believe that he was suffering to such an extent that he lacked the capacity for even complicity.)