AUSTIN, Texas — U.S. leaders say Vladimir Putin used a familiar cyber playbook to “muck around” in the midterm elections last month, but intelligence officials and key lawmakers believe a much more sinister, potentially devastating threat lies just down the road — one that represents an attack on reality itself. Policy insiders and senators of both parties believe the Russian president or other actors hostile to the U.S. will rely on “deep fakes” to throw the 2020 presidential election cycle into chaos, taking their campaign to influence American voters and destabilize society to a new level. The eerie process, which...