Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I say who wrote them.
Samuels. 2000. The gubernatorial coattails effect: Federalism and Congressional elections in Brazil. JOP.
Cox (1997, 1999) has pointed out that national party systems are formed in two steps: (1) district-level phenomena create the district's number of parties, and (2) broader phenomena determine the degree of linkage/aggregation into a national party system. In Brazil, linkage is terrible. Though districts average 3.3 ENPP, Brazil (at the national level) averages 6.3, an astronomic number. Why?
Unlike in the US, legislative coattails are not based on the president, they are based on the state's governor's race. (Recall that all federal legislators are chosen from each state's single, at-large district, which overlaps perfectly with the gubernatorial district.) Thus, candidates for Congress align themselves with a statewide (and single-state) coalition led by a gubernatorial candidate--and this gubernatorial candidate's success determines the state's Congressional vote as well. As a result, "congress is filled with many poorly nationalized parties, [due to] the 'gubernatorial coattails effect'" (p 241).
Congressional candidates need to attach themselves to a larger movement, so that this larger movement can serve as a 'shortcut' that helps undecided voters decide to vote for the candidate.
Because they are loyal not to the president's program, or to a majority in congress, but to the state governor who helped them win. And if they tick off that governor, they might not be invited back into the governor's coalition next time around. And this is why even members of the president's party cannot be counted on to support the president--their first loyalty lies with the governor.
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