For years, researchers, activists and government officials have pointed to evidence that Exxon Mobil had in-depth knowledge of the risks of fossil fuels while misleading the public about the severity of climate change.

During the Democratic forum on climate change on Wednesday night, that issue got a prime-time airing. And Senator Kamala Harris of California got applause from the audience when she answered a question about climate “disinformation” spread by corporate interests.

“What would you do?” Erin Burnett, a CNN moderator, asked. “Would you sue them? Would you sue Exxon Mobil?”

“I have sued Exxon Mobil,” Ms. Harris replied.

Not quite.

An investigation, yes. A lawsuit, no. In 2016, The Los Angeles Times reported that Ms. Harris, then California’s attorney general, had begun an investigation into “whether Exxon Mobil Corp. repeatedly lied to the public and its shareholders about the risk to its business from climate change — and whether such actions could amount to securities fraud and violations of environmental laws.”