I'm going to recommend two books for you. The first is Michael McGerr's The Decline of Popular Politics: The American North, 1865-1928. The second is Glenn Altschuler and Stuart Blumin's Rude Republic: Americans and Their Politics in the Nineteenth Century.

The first book is a relatively standard look at the topic. It was written in 1986, and it promotes the orthodox view: In the South, voter turnout dropped as a result of Jim Crow laws passed (and subsequently enforced) by racist legislatures that came into power after the end of Reconstruction. In the North (a term that encompasses the West as it grew), the case is more interesting and complicated.

McGerr suggests that involvement declined as partisanship declined. Rather than being an emotional, passionate practice with parades, flag-raisings, picnics and events, politics turned into a thinking-man's game, devoid of emotion, passion and celebration. Anticorruption campaigns, an end to the spoils system and the creation of a bureaucratic meritocracy meant there was less tangible benefit to politics as well, which also turned people away.

Robert Wiebe and William Gienapp (as well as McGerr) have followed this traditional view that you appear to hold. They find politics in everything during the 19th century.

Altschuler and Blumin take a parallel but contrarian view of the situation. They contend that turnout is a myth and people might have voted, but they were just as disengaged as they were in the 20th century. They argue that merely looking at voting statistics is misleading, given the widespread ballot-stuffing and election manipulation we know took place in the 19th century. Only once the Australian (secret) ballot was standardized across the United States did we get a "true" measure of turnout.

Furthermore, they argue that just as many people were "very involved" as there are today (proportionally speaking), but what we've lost are the folks who attended elections for free drinks, gifts and the chance to win a reliable job in the new government.