Bureau of Justice Statistics (BJS)
The BJS provides a wealth of crime and criminal justice data compiled by the U.S. government via a variety of data-collection programs. The BJS also provides various tools to produce and download tables on topics such as crime rates, crime victimization, and corrections populations. Many of the BJS' data collections are available via the National Archive of Criminal Justice Data.
Center for Empirical Research in the Law (CERL)
CERL, which is hosted at Washington University's School of Law, "supports the application of sophisticated empirical methodology to legal studies research." Its various projects cover topics such as judicial ideology measures, Supreme Court opinions, and even a legal encyclopedia of popular music.
Judicial Elections Data Initiative (JEDI)
The JEDI ( ... ) project at Washington University's School of Law is devoted to collecting "data on elections to state courts of last resort from 1990 to 2010 in an effort to facilitate replication efforts and to stimulate new research in the area of state judicial selection."
Judicial Research Initiative (JRI)
The Judicial Research Initiative at the University of South Carolina has brought together multiple datasets that are of interest to students of judidical politics, covering topics such as attributes of court cases and justices at various levels of government and measures of judicial ideology. Harold J. Spaeth's data on the Supreme Court may be of particular interest. The Spaeth Supreme Court data also reside at http://supremecourtdatabase.org/
National Archive of Criminal Justice Data (NACJD)
The National Archive of Criminal Justice Data, which is hosted at the ICPSR, is an extensive archive of data about crime and the operations of the criminal justice system. Please note that many NACJD data collections are now restricted and available via application only.
American Fact Finder
The American FactFinder is an extensive source for census statistics from the U.S. Census Bureau. Users can create data tables from the Decennial Census (2000 and 2010), the American Community Survey (2000-present), and the Economic Census (1997, 2002, 2007, and 2012) and download those tables into spreadsheet files. See http://factfinder2.census.gov/faces/nav/jsf/pages/what_we_provide.xhtml for a list of available data collections.
Economic Policy Uncertainty Index
The Economic Policy Uncertainty Index project attempts to quantity economic uncertainty created by macroeconomic policy by coding media coverage, tax codes, and economic forecasts. The data are available in monthly increments.
Federal Reserve Economic Data (FRED II)
FRED II includes time-series data for variables such as GDP, interest rates, exchange rates, consumer prices, and monetary aggregates, and financial indicators. There also also add-ins available to access and use FRED data in Excel, Stata, Stata (again), and R. Most of the data are from the 1950's onwards, though some series extend back prior to WWII. Note that each of the district banks within the Federal Reserve system collects data and constructs indices on conditions within its district as well.
Social Explorer provides quick and easy access to current and historical census data and demographic information. Its contents include the U.S. Census from 1790 to 2010, annual updates from the American Community Survey, data on religious congregations for the United States for 2009, decennial religious congregation data for 1980-2010, and carbon emissions data for 2002. Users can create reports and maps at various levels of geography, including counties, Census tracts, Census block groups, and zip codes, depending on data availability. Social Explorer is also available via Databases at Emory.
American National Election Studies (NES)
The National Election Studies series is one of the premier sources for data on voting behavior and political attitudes in the post-WWII United States. Upon registering (which is free), users can download data and SAS/SPSS/Stata program files and documentation for any of the various NES studies. The SDA Archive at UC-Berkeley also allows users to access recent versions of the NES and extract particular variables from them for those who have specific questions in which they are interested.
Congressional Quarterly Voting and Elections Collection
The CQ Voting and Elections Collection brings together a great deal of data on election results in the United States at both the federal and state levels. This resource is also available via Databases at Emory.
OpenSecrets is a watchdog group with an extensive clearinghouse of data on campaign contributions and lobbying. In addition to its main site, OpenSecrets also has an Open Data Initiative in which users can register to download bulk amounts of OpenSecrets' data for additional analysis.
Policy Agendas Project
UT-Austin's Policy Agendas Project is an ambitious undertaking that strives to "[collect and organize] data from various archived sources to trace changes in the national policy agenda and public policy outcomes since the Second World War." The data collections cover items such as Presidential State of the Union Addresses, roll-call votes and committee hearings in Congress, federal spending, and the priorities and "moods" of the public.
Roper Center for Public Opinion Research - Exit Polls
The Roper Center is one of the country's premier centers for polling data. Amongst (many) other items in its holdings, Roper has an extensive collection of exit polls for federal elections and some state elections. This resource is also available via Databases at Emory.
VoteView, which is a project of Dr. Keith Poole, provides freely-available Congressional data for roll-call votes, party polarization, and ideology scores. See https://legacy.voteview.com/ for data from earlier iterations of the project.
General Social Survey (GSS)
The GSS measures public opinion in the United States on a wide variety of topics of interest to social scientists. The survey, which began in the early 1970's, provides a biennial perspective on American attitudes toward government, life, race, religion, and other social issues. The link here is to the GSS homepage within the National Opinion Research Center. Sites where researchers can extract and download specific variables of interest are listed here. The SDA Archive at Berkeley also holds GSS data from 1972 to 2016 in an interface that allows for basic on-line data analysis and the creation of subsets of GSS data. GSS data are also available via the Roper Center.
The Policy Mood data, which have been assembled by James A. Stimson and K. Elizabeth Coggins at UNC-Chapel Hill, " is a time series measure of public support for government programs on the liberal-conservative continuum." The data cover the years 1952 onwards. An issue-specific variant of the Policy Mood data is available via the Policy Agendas Project (see above). Mood data are also available via James Stimson's page at UNC.
Roper Center for Public Opinion Research
The Roper Center is one of the country's premier centers for polling data, with holdings dating back to 1935. While the bulk of the Center's data are for national polls, it also includes many state-level polls as well. The iPOLL interface may be of particular use because it allows users to search through surveys at the question level. Roper also has a large compilation of Presidential approval ratings. The Roper Center is also available via Databases at Emory.
Measuring American Legislatures
The Measuring American Legislatures project is an effort to code the ideological orientations of both state legislatures and individual state legislators. Data from the project are available via the Dataverse of Boris Shor, who is one of the project's investigators.
National Institute for Money in State Politics (NIMSP)
NIMSP is a watchdog group with an extensive clearinghouse of data on campaign contributions and lobbying, with a focus on elections at the state level. Bulk downloads of data are also available via http://sunlightlabs.github.io/datacommons/bulk_data.html.
Partisan Balance of State Government
Carl Klarner at Indiana State University has assembled various datasets on state politics, including data on outcomes of state elections and partisan control of state legislatures and executives. You can also access Klarner's data via his Dataverse.
State Ideology Data
The State Ideology Data, which have been assembled by Richard C. Fording at the University of Alabama-Tuscaloosa, measure the ideological leanings of both the public and political leaders for the individual states. The data cover the years 1960 onwards.
State Politics and Policy Quarterly Data Sources
This site has various collections of state-level data for the United States. The Practical Researcher Data provide time-series data on political, judicial, economic, and social variables. Depending on the variable, the data coverage is from 1975 to 2006. There is also a collection of individual datasets on particular topics such as partisan balance of state governments. SPPQ also has a journal replication archive with other data relevant to the study of state politics.