Originally posted by Toad
Is there a belief that everything can be modeled correctly?
You are dealing with human voters here, after all.
Realistically speaking, you can't hope to model everything accurately. Humans sometimes act in very random ways that you can never model. However, when humans act in systematic ways, you do hope to capture some of that through modelling.
If presidential elections are actually random events, then no amount of modelling could ever predict more accurately than just flipping a coin. However, we know that some systematic processes drive how people vote at the individual and aggregate levels. Disagreement over the degree to which these factors matter and, more importantly, how to operationalize many of the concepts lead to different results and sometimes inaccurate findings.
Case in point: How do we measure "the state of the economy?" That's a theoretical concept that we potentially measure using any number of indicators. Do we use GDP growth? Stock market growth? Consumer confidence? Some combination of those? Better political scientists than Holbrook demonstrate quite convincingly that different operationalizations lead to better predictions (c.f. Bartels and Zaller 2001; Lewis-Beck and Tien 2001).
Lewis-Beck and Tien, for example, argue that voters apply retrospective and prospective evaluations of candidates differently for incumbents and challengers. They measure the "state of the economy" through GNP change, leading economic indicators, and the Dow Jones with an interaction between incumbency and GNP. This interaction indicates an unequal relationship between economic evaluations of the incumbent president and economic evaluations of challengers.
-- Todd/Leviathn