Another Fine Mess for Trump
The problem with the premise: it's not Donald Trump's mess, it's ours. Or, in other words, he always made his messes become somebody else's - his "best" lenders, his "best" investors, his "best" lawyers, his "best" fixers, his "best" employees, his "best" friends, his "best" wives...
The first problem with his messes: He is a "Do you know who I am?" kind of "celebrity" and a narcissist, so "rules" (and often laws) simply do not apply to them. He was born into money and he resents that nobody believes he is "self-made" and possibly not even a billionaire (if you take the debts into account) so he compensates by imagining, believing and overstating the case for his "greatness," i.e., "worth $10 billion or more, loaned $1 million from parents, "very stable genius" etc. He is and always has been "entitled" - so much for the faux populism that his beloved "uneducated people" are buying. Like all good con men, he knows his unsophisticated audience and customers.
The second problem goes beyond his personal messes" and has to deal with the systematically strengthened to the point of being (by design) practically unchecked powers of the presidency and while we may often know "who is watching the watchers" we don't have a scrip / prescription / set of enforceable rules when watchers find improprieties or illegalities.
Having DOJ or FBI investigate President is a folly, because while we can keep telling ourselves that it's "independent"... it really isn't, it's part of executive branch. Neither, it turns out an "independent" Federal Reserve, is safe from corrupt Chief Executive. Thinking something is "technically independent" because that's how it always was "the norm" doesn't make them truly or "actually" independent by law.
The real problem with this mess is that neither Constitution nor Congress(es) usually prescribe what the specific civil or criminal penalties are in cases of high government officials and especially presidents flouting the law. And don't say "impeachment" because it's an (increasingly) ineffective and weak political tool dependent on vague standards and more importantly political and economic state of the nation / state, and the will of the majority of people and super-majority of Congress rather than a specific set of enforceable civil and criminal penalties. Combined with powers of pardons and other indulgencess given to President, impeachment has become all but a threat of a political theater which, sadly, has been established during Clinton's term.
Trump knows it and invites it. It only makes him look like a "victim" of a political "witch hunt" so it would only scare him if it will uncover his gross financial misdeeds, like money laundering - in which case he should be criminally prosecuted for it and/or civilly fined and bankrupted. He was proud to screw his banks, investors, contractors, "students" as long as he could continue doing what he's been doing at minimal cost to his pockets and his freedom. So impeachment that fails to remove him doesn't scare him, he'll wear it as a badge of honor and point to it as a reason for his loss in 2020 (if dems will not shoot themselves in a foot again and find someone even more lethargic and self-unaware than Hillary.
"He [Nixon] left. I don't leave", i.e. "Deal with it," "Make me," "Whatcha gonna do, impeach me? Go ahead, make my day!"
By these standards, the otherwise third-world country like Brazil and some others in Latin America and Asia that tried and put in prison their former "dear leaders" are way ahead of the U.S.