Adopted in 1913, the 17th Amendment to the US Constitution requires that members of the US Senate be elected by citizens of their respective states. This article is concerned with understanding what factors have influenced Senate election outcomes and how their effects have changed over time. I focus on three independent variables, state partisan composition, incumbency, and national partisan tides, and then develop a model of Senate election outcomes that allows their effects to change from election year to election year. The results reveal substantial variability and lead to a new and detailed understanding of Senate elections in the United States. The significance of the findings is further demonstrated by showing how the effects of the independent variables have affected actual election outcomes.