Elections: Think Tank Reports / Studies

"The Brennan Center for Justice at New York University School of Law is a non-partisan public policy and law institute that focuses on the fundamental issues of democracy and justice." [About] This page provides links to the Brennan Center's research on voter ID, proof of citizenship, and in-person voter fraud.
See also: NCSL's Voter ID State Requirements listing changes and proposed changes to Voter ID requirements, by state.

The Brookings Institution is a nonprofit public policy organization based in Washington, DC, known for high-quality, independent research.

Although this has an international, country-level, focus, it also includes some sub-national US data.
About the IFES: "The International Foundation for Electoral Systems (IFES) supports citizens’ right to participate in free and fair elections. Our independent expertise strengthens electoral systems and builds local capacity to deliver sustainable solutions." Includes:

• Public Opinion Surveys
  "Through IFES' Applied Research Center, public opinion research approaches are developed for each country context, with no one-size-fits-all approach. This method has unearthed findings across the globe on voter education, elections, government, social status and much more.
  An example of IFES' long-standing work in public opinion is the annual Ukraine survey. IFES has conducted 19 surveys in Ukraine..."
• Election Resource Center
  "The ERC contains thousands of key electoral documents – including ballots, country reports, electoral codes, civic education posters and more.