This paper studies how electoral institutions shape candidates' digital campaign strategies, exploiting Brazil's 200,000-voter runoff threshold in a regression discontinuity design. 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 document three patterns of digital campaigning: near-instantaneous ad deployment, a strong preference for broad demographic reach over micro-targeting, and a sharp escalation in advertising intensity as election day approaches. Runoff-eligible candidates reduce their digital boosting budget share by approximately 4 p.p in the first round, with the effect concentrated in the final weeks and driven entirely by competitive candidates. No other expenditure category responds to the threshold. This strategic reallocation operates through scale rather than content: candidates above the threshold accumulate approximately 866,000 fewer impressions without altering the format, duration, or cost structure of individual ads. Consistent with this timing behavior, we find that an increase of BRL 100,000 in digital boosting is associated with a 5 p.p increase in vote share, with total reach — not content — driving electoral returns. These findings reveal digital advertising as a strategically flexible instrument whose near-zero marginal deployment cost enables intertemporal allocation decisions that traditional media cannot accommodate.