Long-running series of survey data on voting, public opinion, and political participation. 1960 - 2012.

Online analysis of American National Election Survey from Berkeley's Survey Data Analysis. This is a great place to start if you want to explore questions asked, get initial frequencies, or select individual questions across surveys for data download.

Supplementary Empirical Teaching Units in Political Science (SETUPS) are modules designed for use in teaching introductory courses in American government and politics. The modules are intended to demonstrate the process of examining evidence and reaching conclusions in a way that stimulates students to think independently and critically, with a deeper understanding of substantive content.

Datasets available here include his Federal Elections Project (census to precinct-level elections data), Candidate Quality and U.S. Senate Election Data,
Congressional District Demographic and Political Data, and the Southern Politics Project data.

Precinct-level data on election returns, demographics, and political behaviors from 1984-2000.

Data on over 1 million campaign ads aired for the 1998 through 2008 elections.

The successor to the Wisconsin Advertising Project, this project tracks and analyzes advertising in federal and state elections since 2010.

"The resulting database contains over 100 million political contributions made by individuals and organizations to local, state, and federal elections spanning a period from 1979 to 2012."

Project by Internet Archive to capture all political advertising during 2016 campaign in 20+ states.

Directory to various collections of advertising from political campaigns, 1952-2008.

Printed ephemeral campaign materials for the Los Angeles area for the past 100 years, collected by UCLA.

"Largest and most comprehensive political advertising collection in the world." Over 90,000 commercials back to 1930s. While these are not digitized, you can request to borrow a small number of ads.

Web archives related to a number of international elections and the U.S. 2000 - 2008 elections. (Archived.)

Presidential campaign commercials, 1952-2016.

Directory to campaign ads in the last couple of decades, particularly California.

Directory to online collections of political campaigns (plus an extensive bibliography).