This paper studies how electoral rules shape candidates' digital campaign strategies, exploiting Brazil's 200,000-voter runoff threshold in a regression discontinuity design. We test whether candidates who anticipate a potential second round reallocate their digital spending intertemporally, preserving resources for the runoff at the expense of first-round exposure. We combine official campaign finance records from the 2024 municipal elections with high-frequency Instagram advertising data from Meta's Marketing API — 57,677 ads matched to 540 candidates. We show three patterns of digital campaigning: ads are launched within hours of being created, candidates favor broad demographic reach over micro-targeting, and advertising intensity escalates sharply as election day approaches — serving as a tool for rapid and broad-reach voter outreach rather than for sustained or targeted communication. Runoff-eligible candidates reduce their digital boosting budget share by approximately 4 p.p. in the first round — roughly USD 4,200 less in digital expenditure — with the effect concentrated in the final weeks and driven entirely by competitive candidates. No other expenditure category responds to the threshold. The reallocation operates through scale rather than content — candidates reach fewer voters without altering the format, duration, or text of individual ads — consistent with digital advertising serving as a strategically flexible instrument whose near-zero marginal cost enables intertemporal allocation decisions that traditional media cannot accommodate.