Charlie Kirk, one of America’s leading conservative activists and a close friend of President Donald Trump’s son Don Jr., has been murdered. We do not yet know who did it, or why. We do not know how the Trump administration will respond.

What we do know, however, is that there are good reasons to be afraid.

When a prominent political figure is assassinated, the very foundations of democracy come under attack. Democratic politics is, at its heart, a system for containing political violence: a system for resolving the inevitable deep disagreements between citizens without anyone resorting to bloodshed. It works when all major factions believe that the others are committed to following the rules of the peaceful political game; when that belief erodes, it breaks down.

In the past, the American democratic consensus has been strong enough to survive assassination attempts. Some, like the murder of Martin Luther King Jr., tested its bonds but didn’t break them. Others, like the assassination of John F. Kennedy Jr., may actually have strengthened them by creating a sense of shared grief and........

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