Russian interference tended to focus on telling the targeted group to fear and distrust the other side. Posts that targeted Republicans, for instance, portrayed Mrs. Clinton as a threat to the Constitution and public safety, and in one memorable example said that her election would be a victory for Satan himself.

In taking this approach, the Russians were merely riding a trend that has been building for decades. Since the 1980s, surveys have found that Republicans and Democrats’ feelings toward the opposing party have been growing more and more negative. Voters are animated more by distrust of the other side than support for their own.

This highlights a problem that Lilliana Mason, a University of Maryland political scientist, said had left American democracy dangerously vulnerable. But it’s a problem driven primarily by American politicians and media outlets, which have far louder megaphones than any Russian-made Facebook posts.

“Compromise is the core of democracy,” she said. “It’s the only way we can govern.” But, she said, “when you make people feel threatened, nobody compromises with evil.”