Mr. Nimmo’s goal is to spot disinformation early — essentially, to stamp out the fire before it spreads.

His techniques have changed as his adversaries have become more cunning. Because Facebook, Twitter and YouTube are now policing their platforms more aggressively, he is less able to rely on obvious clues like masses of automated Twitter posts and fake Facebook accounts.

So Mr. Nimmo has started looking for clues in obscure areas of the internet, like German news sites that accept unverified user-generated content and Iranian video-sharing services. Websites like Reddit, Medium and Quora are becoming popular places to create fake accounts and plant disinformation and leaks.

“Every time we catch a threat actor, you can bet that the other ones will change their tactics to try and keep ahead,” he said.

More interference is coming in the 2020 campaigns, Mr. Nimmo said. He said he was particularly worried about a “hack-and-leak” operation like the one in 2016 when Russian operatives took information from the Democratic National Committee’s servers and got it published online. Loaded with juicy and accurate information, such leaks go viral on social media and can be irresistible to the news media.

Mr. Nimmo’s path to disinformation research was not an obvious one. An Englishman who studied literature at Cambridge University, he worked as a scuba diving instructor in Egypt, as well as a travel writer and journalist in Europe. In 2007, while reporting on violent demonstrations in Estonia for Deutsche Presse-Agentur, he was head-butted by a protester, breaking his nose and leaving it off center still today.

In 2011, he began working at the North Atlantic Treaty Organization as a press officer. While there in 2014, he saw how Russia had worked to muddy perceptions of its invasion of Crimea that year, including misrepresenting Russian soldiers as “local self-defense forces.”