Which digital marketing tactic showed the highest ROI for presidential campaigns in the 2020 U.S. election cycle according to post-election analysis?

The 2020 U.S. Presidential Election saw unprecedented digital marketing strategies deployed by both major campaigns. With traditional rallies limited by the pandemic, campaigns had to innovate their outreach. This trivia question focuses on which digital marketing tactic proved most effective in terms of voter engagement metrics, fundraising success, and conversion to actual votes according to post-election analytics from the Cambridge Digital Politics Research Group.

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Micro-targeted social media ads using voter data analytics
Celebrity endorsements on Instagram and Twitter
Traditional email marketing campaigns with donation requests
Campaign-specific mobile apps with gamification elements