I don't know if one can pinpoint a specific year or presidency as to when that became impossible. I think it's been a gradual process as the federal government began assuming more power unto itself, thus leading to national politics becoming more important than they should be. As issues such as abortion and the size of government got taken away from the local level and all became issues of national import, national politics became all consuming and the stakes started becoming too high.
Now that the stakes have all been turned up to 11, much as they were in the 1850s, and where people view national politics and national elections as existential events, coupled with the Internet and SM, political violence has to be expected for anyone with eyes to see and ears to hear.
The lefties began this most recent (Trump era) downward spiral into societal shunning and political violence. They will not be heard now to object when such is visited upon them.