1. The argument that Chait's thesis can be safely disregarded because of his past support of the Iraq war and his incorrect predictions about Trump is logically fallacious (ad hominem). Let's judge his ideas based on the merits, not on his past.
2. Agree on the whole that nominating Sanders would be a disaster. Americans are not going to go in for a guy who continues to insist on calling himself a socialist. The man has not even strongly disavowed his past support of state ownership of the means of production (only his surrogates have put out quiet messages to this effect at times). For a guy who insists that he only cares about the issues, this is a weird rhetorical albatross to voluntarily wear.
3. His socialist policies are even more problematic. Americans are wary of things that sound too good to be true (in general). Free college, free healthcare, free leave, etc with no major increase in the tax burden on the middle class sounds too good to be true, and it is in fact. The social democracies of Scandinavia that have these policies while maintaining their democracy are able to do so by paying exorbitant tax rates and having an extremely high cost of living. There is no free lunch. Americans love the idea of medicare for all, right up until the point when you start discussing how you're going to pay for it.
Sanders' team seems to understand how unpopular his policy proposals are more and more as the possibility of his nomination increases. AOC recently remarked that we shouldn't be scared of Bernie's big ideas because they are unlikely to pass through congress. She's right, but it's a pretty bad look for your campaign when you're trying to draw in support by undermining your signature policy idea.
4. The discussion about health on here is not great, guys and gals. This is America; we are all free to decide who to vote for based on any fucking thing we like. Health, and its close correlate, age, are perfectly defensible criteria for judging candidates. Bernie just had a heart attack; it's obviously true that this increases his chances of dying in the next eight years, especially given his age. Joe Biden clearly has trouble maintaining his eloquence over long stretches of time (just watch one of the recent debates). Trump also clearly has difficulties with language at times. I think if we're honest with ourselves, none of these statements are controversial. I agree that we cannot definitively diagnose the president/biden with dementia based on these snippets of information, but it is also okay to take this information into consideration when deciding whom to vote for. Buttigieg, Klobuchar, Warren do not seem to have these problems. It's no accident that they're younger. That doesn't mean that the older candidates must be disqualified, but it is reasonable to count this in their favor.
5. The stuff about Trump fanboys on here is so tiresome. This is a sub plot on every thread on this forum. I propose that we stop having such long fights about this. Let's all recognize that we all have our biases, we all have our principles and try to judge our arguments on the merits without making everything personal. I find it weird how both sides treat each other with such contempt, mostly because it's so self defeating in politics. I didn't vote for Trump, but I'm not going to shit on someone just because they did. I'm going to try to convince them to vote for my candidate. How will I be able to convince them if I belittle them at every turn? The truth is that people have their reasons for voting the way they do. I may disagree, but I'm not going to do any good by shitting on them about their choices.
6. Someone made a point about people who wish the president does not succeed. The same person said he views such people as unamerican, and thus that he will treat them with contempt and prejudice.
There is a grain of truth here, because I agree in general that nobody should wish the country does badly just so their side gets to have power. However, I am making a distinction here between the country and the president. I want the economy to do well, but that doesn't necessessarily mean that I want all of Trump's policies to pass. I also think it's possible for a person to hope that things go badly in the short term because in the long term the country be better for it. Perhaps I'm giving such folks too much benefit of the doubt. As an extreme example, if a president eventually becomes dictator for life, it probably would have been better if the economy had dipped a little bit during his democratically-elected period in order that he be kicked out of office before consolidating his power.
Calling people unamerican for thinking something and saying you will treat them with contempt and prejudice is similar to the self-defeating attitude I outlined above. I would only hope that everyone on here try to give everyone else the benefit of the doubt and have faith that we are all just trying to advocate for what we see as best for the country. I think this attitude is the only way we can resurrect a productive civil discourse.