SAN FRANCISCO — Before the sun came up on Oct. 31, Renee DiResta sat in bed in her pajamas and logged into a virtual war room.

For years, Ms. DiResta had battled disinformation campaigns, cataloging data on how malicious actors spread fake narratives online. That morning, wearing headphones so she wouldn’t wake up her two sleeping children, Ms. DiResta watched on her laptop screen as lawyers representing Facebook, Google and Twitter spoke at congressional hearings that focused on the role social media played in a Russian disinformation campaign ahead of the 2016 election.

Ms. DiResta knew the lines of questioning inside and out. Along with a handful of people with a similarly obsessive interest in mapping data across social media, she had helped prepare congressional staff members ahead of the hearings. That morning, they gathered in a dedicated channel on the Slack messaging app to watch and listen for answers to questions they had been asking for years.

“We were monitoring closely to see when the companies gave misleading or partial answers so that we could follow up,” said Ms. DiResta, 36, who became immersed in disinformation campaigns in her spare time outside of her job as a founder and head of marketing at Haven, a shipping technology company.