President Trump’s chief of staff told her not to bring it up in front of the president.
Ms. Nielsen left the Department of Homeland Security early this month after a tumultuous 16-month tenure and tensions with the White House. Officials said she had become increasingly concerned about Russia’s continued activity in the United States during and after the 2018 midterm elections — ranging from its search for new techniques to divide Americans using social media, to experiments by hackers, to rerouting internet traffic and infiltrating power grids.