Table 1.

Theoretical justification for the US state-level variables: political metrics, health outcomes, and socioeconomic covariates.

State-level variable	Theoretical justification	Additional considerations
Political metrics	1. Political ideology of elected representatives. Elected officials campaign to win positions of power and legislate based on their political ideology. Political ideologies, while complex, in the United States typically are categorized as ranging from conservative to liberal, in relation to both social and economic positions. Political ideologies can affect population health and health inequities via their material impacts on people's living and working conditions and on their economic, social, political, civil, and cultural rights.	Politicians’ legislative voting records (ie, roll-call votes) provide public data on their political positions and can reflect their willingness to use state power to implement their political ideology. In the United States, members of Congress (US House and Senate) legislate to shape federal policy, including its impact on states and resources for constituents in their states. A political ideology score based on roll-call votes of US members of Congress is available (DW-Nominate) and is used in numerous social science studies, but rarely in population health studies.23,31
	2. Political party concentration of power. Political parties provide the resources and organizational means for politicians with a similar political ideology to enact their legislative agenda. Achieving political party concentration of power, in order to have a sufficient number of votes to pass legislation and override any vetoes, is a core objective. The likelihood of particular policies to impact population health depends, in part, on the power of political parties to enact the legislation they advocate.	In the US political system, political party concentration of power can be measured in relation to whether a single party controls the executive and legislative branches of government. At the US state level, this involves control of 3 seats of power (also referred to as a “trifecta”): governor (executive) and each component of their bicameral legislatures (eg, House and Senate), with the 1 exception of Nebraska (which has a unicameral legislature).32 Trifecta data are used in numerous social science studies, but rarely in population health studies.23,32,33
	3. State policy index. State policies are the political enactment of elected officials’ political ideologies and the government agencies they control and whose budgets they set. These policies, enacted over time, singly and together, shape the conditions of life for people and ecosystems within their state (and sometimes neighboring states, as per the case of environmental pollution), thereby affecting people's health.	Beyond the direct health impacts of specific policies (eg, Medicaid expansion), both public health and social science studies increasingly are investigating impacts of policy “bundles,” which reflect a shared underlying political ideology and base of support.2,10,27,34-36 Some indices of state policies focus on multiple policies in a specific policy domain (eg, health care) and others consider policies across multiple domains (eg, social and economic policies); while many are cross-sectional and cover a limited set of years, some include decades of policy data to create dynamic measures of state policy indices.27,37
	4. Voter political lean. Voters’ political ideologies can affect which political parties and politicians they vote for and hence which state policies are or are not enacted. The partisan identification of voters is the basis for measuring voter political lean, which typically is measured as the percentage difference between the vote for a particular party in a particular area (eg, state or political district) vs in the national vote. Voter political lean is often interpreted as being a measure of voter political ideology. One impact of voter political lean on health is via the politicians (and other elected officials) who are elected vs defeated, as well as the ballot initiatives passed vs defeated; others may be via pathways involving interpersonal relationships between members of the electorate.	Voter political lean is a metric increasingly used in public health research since 2020, in research focused on COVID-19 pandemic and political polarization.14-23 Key caveats are that voter political lean (1) is based solely on who votes, noting that only 66% and 46% of US eligible voters, respectively, voted in the 2020 presidential election and the 2022 midterm elections,38 and persons least likely to vote are concentrated among politically, socially, and economically marginalized sectors of society who also are at higher risk of poor health1-3,38,39; (2) does not reflect views of persons legally not permitted to vote (eg, ex-felons, noncitizens, young people aged <18); and (3) ignores gerrymandering (ie, legislators’ manipulation of district boundaries to give unfair advantage to population groups deemed likely to elect them), which deliberately diminishes the votes of those targeted adversely by gerrymandering and affects which policies are enacted.5,28,40,41
Health outcomes	Guiding selection of 8 chosen health outcomes is that they (1) span the life course, enabling detection of associations with political metrics across all age groups; (2) are quickly responsive to contemporaneous exposures (ie, have a short etiologic period); and (3) are important public health indicators, with all except 1 (vaccination for the newly emergent disease COVID-19) designated as either “leading health indicators” or “objectives” in 1 or both of the US Department of Health and Human Services agenda-setting national initiatives Healthy People 2020 and Healthy People 2030.29,30	All health outcomes but one were based on state-level data for the entire population; the exception was the state-representative survey data on percentage of adults without health insurance. The specific outcomes selected involve both health status and access to health care. They comprised infant mortality, premature mortality (death before age 65), lack of health insurance among working-age adults (ages 35–64), childhood immunization, flu vaccination among adults aged ≥65, COVID-19 vaccination among adults aged ≥65, food insecurity, and maternity care deserts.
Socioeconomic covariates	We opted to adjust for state poverty rates as a potential confounder, since (1) poverty is associated with the selected exposures and health outcomes and poverty rates vary by state and (2) poverty rates (unlike other socioeconomic metrics, such as educational level or wealth) are rapidly responsive to changes in fiscal policies.42-48	We focused on poverty rates among children (<18 years) and adults aged ≥65 because numerous US safety net programs focus on these age groups and use the Federal Poverty Level to determine eligibility.42-51 We recognize that adjusting for poverty may result in attenuated estimates of association, since it may be on the causal pathway between state policies and health outcomes.

Social epidemiologic, political science, and political sociology theories inform the justifications for the variables selected; for supporting scholarship, see citations 1-8, 21-28. Descriptions of the specific variables used are presented in the Data and methods section, and details on how to access these data and how to construct the variables used are provided in Table S1 and Textbox S1, respectively.