This article examines the electoral consequences of variation in voter turnout in the United States. Existing scholarship focuses on the claim that high turnout benefits Democrats, but evidence supporting this conjecture is variable and controversial. Previous work, however, does not account for endogeneity between turnout and electoral choice, and thus, causal claims are questionable. Using election day rainfall as an instrumental variable for voter turnout, we are able to estimate the effect of variation in turnout due to across-the-board changes in the utility of voting. We re-examine the Partisan Effects and Two-Effects Hypotheses, provide an empirical test of an Anti-Incumbent Hypothesis, and propose a Volatility Hypothesis, which posits that high turnout produces less predictable electoral outcomes. Using county-level data from the 1948–2000 presidential elections, we find support for each hypothesis. Failing to address the endogeneity problem would lead researchers to incorrectly reject all but the Anti-Incumbent Hypothesis. The effect of variation in turnout on electoral outcomes appears quite meaningful. Although election-specific factors other than turnout have the greatest influence on who wins an election, variation in turnout significantly affects vote shares at the county, national, and Electoral College levels.