Of course, the simple answer, and I suggest the correct one, is the 20-year-old shot dead on the Pennsylvania rooftop.
Apparently, the person was registered as a Republican but made a modest contribution to a left-wing organization. There is no evidence that he was other than a lone wolf whose motivation was unclear (as this is written).
So why is this attempted assassination being treated as if it's more than it appears – an isolated action by a single individual?
The immediate and almost instantaneous verdict is this was the product of America's deep polarization where members of each side hate their opposition.
While the divide is certainly real and intense, it can't be blamed for the conduct of a lone wolf.
Of course, the name-calling by Democrat leaders that Trump is a fascist and enemy of democracy causes a hostile reaction from his supporters in the same fashion that the former president's attacks on his foes as enemies of America do.
Leave aside whether either set of allegations are true, they are political speech. Are such statements to be discouraged because hearing them may inflame violent actions? (If, however, such verbal attacks are demagogic it is simply irresponsible to utter them knowing their possible effect on a few of 330 million Americans.)
Yes, many of America's leaders contribute to the polarization of politics in a way that transforms the view of opponents into enemies. That cannot be good for democracy which needs a commitment to shared values to survive.
But America, our system of government, is not responsible for the few loons who choose to kill in support of their apparent political objectives.
[Postscript-- the positive of the universal condemnation of
America's deep political division, implicitly the cause of the shooting, is the
likely dampening, if not reversal, of bipartisan anger. That would be a good thing.