Still, many people on both the left and the right have been skeptical of the notion that Russia’s industrial-scale trolling campaign made a significant difference in Trump’s election. In February, the special counsel Robert Mueller indicted 13 Russians connected to the Internet Research Agency, a Russian trolling operation based in St. Petersburg. Shortly afterward, National Review’s Rich Lowry scoffed that the “Russian contribution on social media was piddling and often laughable.”

That month, Adrian Chen, who reported on the Internet Research Agency for The New York Times Magazine, appeared to minimize its political impact in an interview on MSNBC. He called it “essentially a social media marketing campaign with 90 people, a few million dollars behind it, run by people who have a bare grasp of the English language and not a full understanding of who they’re targeting.”

At a conference in Germany in July, I met Denis Korotkov, a brave Russian journalist who has reported extensively on Yevgeny Prigozhin, the oligarch behind the troll factory, who was included in Mueller's February indictments. Speaking through an interpreter, he expressed incredulity at the idea that the misinformation being spread from St. Petersburg could have changed the direction of American history.

But it looks increasingly as though it did. On Monday, the Senate Intelligence Committee released two reports it commissioned about the nature and scale of Russia’s social media disinformation campaign. One was produced by New Knowledge, a Texas cybersecurity company, along with researchers at Columbia University and Canfield Research; the other was written by researchers at Oxford University’s Computational Propaganda Project along with Graphika, a company that analyzes social media. They were based on (incomplete) data turned over to Congress by several major social media platforms, and they suggest the campaign was more extensive and sophisticated than has been previously understood.

Russian propaganda, one of the reports found, had about 187 million engagements on Instagram, reaching at least 20 million users, and 76.5 million engagements on Facebook, reaching 126 million people. Approximately 1.4 million people, the report said, engaged with tweets associated with the Internet Research Agency. “The organic Facebook posts reveal a nuanced and deep knowledge of American culture, media and influencers in each community the I.R.A. targeted,” it said.