The challenge I have faced on this point is one of context. Slavery was already going to make the job difficult, and even if Lincoln got there first, it still wouldn't have been easy. Context determines the magnitude of the mistakes you can make, and the costs. So, imagine Trump in 1856. How much worse would he have been than Buchanan? Boggles the mind to wonder, doesn't it? (And here's the scary one: Imagine Trump in 1962. And we awoke the next morning to discover that we were all Gregor Samsa.)
In contrast, does modern society make something like the Civil War just so unlikely that we have to adjust our expectations and evaluate accordingly? By modern standards and in modern context, does encouraging violence against the press, and then blaming the press when someone tries to bomb CNN constitute being worse than Buchanan, even if the death toll is lower (so far, zero), just because... come on, Donny, how hard is this, you worthless fuckwit?!
A few points. First, what would it take for Trump to take the title, in my estimation? An example would be responding to this stuff by declaring martial law. "Martial law" is one of those scary-sounding terms from fiction, but in times of war, you can get things like a suspension of, oh, Habeas Corpus, just to pick something vaguely relevant to my Buchanan reference. Other bad things have been done by presidents and "justified" by claims of wartime needs, like the internment of Japanese-Americans during WWII, because fuck FDR. What might Trump do if a bomb goes off? I speculated about political effects yesterday, but I forgot to mention the obvious thing-- declare it a false flag attack as some batshit crazy conspiracy theory, like some in the GOP and its media operation are now doing. Yes, it was obvious that the GOP would do this. They are fucking nuts. Here's an example of how Trump could take the title from Buchanan-- use one of these insane lies, after a bomb goes off, as an excuse to start rounding people up, shut down CNN, or something like that.
I'd like to say that this is completely "inconceivable," but it is only inconceivable in the Wallace Shawn sense of the word. (Did I get you back on board with that one, after my Gregor Samsa thing?) If Trump thought he could get away with shutting down CNN, do you really think he'd resist the temptation? Be honest. He worships Putin, he's totally fine with the brutal torture and murder of Khashoggi… If he really thought he could get away with it?
Is this likely? No. I am posing a scenario under which Trump could take the title of worst-ever from Buchanan. It isn't likely. And that's my point. I can't discount it completely because Trump wants to be an autocratic dictator, as we can see from the fact that he idolizes autocratic dictators who murder journalists, but this is not a likely scenario. My point is that this is the type of thing that would need to happen for Trump to take the worst-of-all-time title away from Buchanan. The Civil War is far enough back in history that understanding it is difficult for modern Americans. It requires reading and understanding just what a useless dipshit James Buchanan was, and connecting the two, but really, the Civil War was really bad, and James Buchanan was really incompetent. It just isn't in your face, on your tv screen or computer monitor, live every day. Your brain isn't wired to handle this rationally. You are using what is called "the availability heuristic." Stop it.
Now, as for the adjustments. I was also prompted in the comment about the concept that the scale of war in modern times is just too high, given modern weaponry, for a repeat. Conflict gets toned down because the cost of fighting is too high. The scale of death in the Civil War was unimaginably high, but regardless, that's not what's stopping a Civil War right now. What's stopping a Civil War right now is that no state is trying to secede. Hey! Look at me, tautologizing like a tautologizer! The geographic and military structures just wouldn't permit anything like that.
Blood in the streets? That possibility, I can't discount, but geographic/military structures make the kind of organized, secession-based civil war not really likely. So, I have to look for a different standard. Yes, Trump is really horrible, and right now, it looks like he is actively trying to tear the country apart. That means something different now than in 1856. The structure of conflict is different, and there would likely be a lot less blood because of the geographic/military structures at least as much as anything else. One could hope for none, but if we're being realistic, the probability of that declines the more demagogic Trump gets, and he started out psychopathically demagogic.
You still have to read your history books to understand just how bad James Buchanan was. Remember, I really hate Trump. I am the Trump-hating-est Trump-despiser of them all. But he's still not the worst ever. You are being confronted with images on your tv with him being horrible, and all you have for the Civil War is, what, that Ken Burns documentary with the sad harmonica and silly accents reading letters? No, really, James Buchanan was the worst.