Context: In November 2017, Maine became the first state in the nation to vote on a key provision of the Affordable Care Act: the expansion of Medicaid.

Methods: This study merged official election results from localities across Maine with Census Bureau and American Hospital Association data to identify characteristics of areas that support Medicaid expansion.

Findings: Places with more bachelor's degree holders more often vote in favor, whereas those with more associate's degree graduates tend to vote against. Conditional on education rates, areas with more uninsured individuals who would qualify for expanded coverage tend to vote in favor, while those with more high-income individuals tend to vote against. Also conditional on education rates, greater hospital employment is associated with support for expansion, but the presence of other health professionals, whose incomes might decrease from expansion, is associated with less support.

Conclusions: Voting patterns are mostly consistent with economic self-interest, except for the sizable association of bachelor's degree holders with support for Medicaid expansion. Direct democracy can shift Medicaid policy: extrapolating to other states, the model predicts that hypothetical referendums would pass in 5 of the 18 states that had not yet expanded Medicaid at the time of Maine's vote.

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