Understanding The Coattail Effect: A Critical Analysis

Causal identification of coattail effects is challenging because popularity shocks typically affect parties in both concurrent elections. Although the presidential coattail effect has been an object of frequent study, the question of whether popular congressional candidates boost vote shares in return for their parties presidential candidates remains unexplored. The coattail effect, a phenomenon where an incumbent president's performance influences the electoral success of their vice president, is a critical yet. The coattail effect is the ratio of the effect of a ban in a concurrent election on support for batasuna over the direct effect of the ban on support for batasuna in the election where it takes place. Hence, to estimate the coattail effect, we need a measure of support for batasuna under the ban.

In this paper, we start by describing the empirical difficulties involved in consistently estimating coattail effects. We then exploit the unique characteristics of the mexican electoral system to recover what we believe to be an unbiased estimate of the magnitude of coattail effects from municipal to state legislative elections.