There is a certain degree of imperialism, racism, and the like in even the standard version of American history, ranging from slavery to the treatment of Native Americans. But I'm wondering what are the "great" Presidential scandals or misdeeds at the level of a Nixon or Bush from past periods of history. I feel like earlier scandals, say your Teapot Dome or generic corruption of the Grant administration, is presented as a nostalgic period in which graft was the only problem. The type of controlling megalomania of a Nixon or Bush seems missing. Is this just because we don't have the records of it? Or maybe I'm missing a scandal?

Presidents from Washington to FDR have been accused of harboring dictatorial tendencies, but have been vindicated by history and I'm not aware of good arguments that they actually sought dictatorial control (though I can understand how contemporary critics thought so). 

The further back that you go, the weaker the Federal government is, so the harder it is to find a President who might have grabbed absolute power. You have to look to the local governments, which is why I mentioned the political machines. You could also look to the Jim Crow system in the south. However, both of those are viewed as "solved problems" and are not considered relevant to modern politics.

One problem is to distinguish between the "primitive morality" of our racist forebears and the nature of the institutions that they lived with. I'm sure that a statist would argue that the society was oppressive, and the state simply reflected the society. Conversely, I suspect that the state is inherently oppressive (structured to allow one portion of society to dominate the rest) and that its existence invites oppression and it is actually an artifact of the oppressive history of our society (i.e. it has no value in a non-oppressive society).

Likewise, political machines and corruption are linked to identity politics and the absence of a professional class of state employees. Now that we've largely solved those problems, we don't have to worry about it, right?

That's the problem with going back too far (or looking to the experience of other counties)--there are just too many differences between them and us to make a strong comparison.

The other "crime" of Bush was his warmongering...so we'd need to find other military adventures where a generally unwilling or ambivalent population was drawn into an unjust or unwise war by the President.  We generally win our wars and decide that they were justified, so this is a bit hard.

There's the escalation of Vietnam, which I could make an argument --at least to liberals--that it was only made necessary by the nature of our political system (being based on intimidation and suspicion). To attack the actions of the Cold War (proxy wars and coups) as being symptoms of the state, we'd basically have to present the entire Cold War (starting with our involvement in the Russian Civil War) as a symptom of the state. That's a hard sell.

Korea and the World Wars are generally considered "good wars", and at most I could argue that the unconditional surrender of Japan (requiring invasion or atomic terrorism)  was not really necessary, and only seemed necessary because of the nature of the political system--but I think that most Americans were on board.

The Spanish war has its questionable  rationale, and was followed by the colonization of Philippines and the neo-colonization of Cuba (and the rest of Latin America). Similarly, there was the colonization of Hawaii.

There was the invasion of Mexico, which prompted Thoroeau's Civil Disobedience and may have been opposed at least by the abolitionists.

I guess one of the big issues is whether we needed to engage in global power politics, or whether we would have been secure if we had just minded our own business.

Although there is a trend to redeem John Adams as President, I think it's possible he could be seen as a President who made a clear bid for what was rightly called out as dictatorial powers.

Discussion of "dictatorial tendencies" is based on a model of how the American system of government works, and therefore what fundamentally undermines it. I view the American government as a class system, where the ruling class is pretty flexible in incorporating outsiders who "buy in" to the system (both economically and morally), and factions of the ruling class vie for electoral support from the general public or middle class. However, there have traditionally been groups that are excluded from power -- blacks and the working poor being two of these groups. While individuals from these groups can join the ruling class, the ruling class reacts violently to any power grab by an excluded group.

Under normal conditions, the factions of the ruling class argue over policies and struggle for power, but each is fine conceding power to the other. However, at times one faction (liberals) tries to incorporate an outsider group, and the other faction (conservatives) feel threatened and a serious fight develops within the ruling class. The weapons traditionally used against the outsider groups are then turned against one faction of the ruling class, leading to the risk of dictatorship.