Incumbency advantage in U.S. congressional elections has been a well-established feature of American politics. Since the late 2000s, this advantage has significantly declined, falling from a longstanding average of 10 percentage points to just 3, as we document using a regression discontinuity design. We show that this decrease was driven primarily by the expansion of mobile broadband. Both Democrats and Republicans were affected, though the decline was initially greater for the party holding the presidency at the time. Mobile broadband disadvantaged incumbents and benefited challengers. It improved voter knowledge of both, increased disapproval of incumbents, and enhanced challengers’ fundraising capacity.