Vice President Kamala Harris and former President Donald J. Trump are carrying out a virtual house-to-house hunt for the final few voters who are still up for grabs, guided by months of painstaking research about these elusive Americans. Inside the Delaware headquarters of Ms. Harris’s campaign, analysts have spent 18 months curating a list of which television shows and podcasts voters consume in the battleground states. Her team has assigned every voter in these states a “contactability...