According to a September 2010 study conducted by the nonpartisan Pew Center for the People and the Press, independents are the largest voting group in the United States, followed by Democrats, then Republicans; that said, more than half of independents formerly identified with one or both of those major parties [source: Pew Research Center]. In a related development, Democratic and Republican rosters have slimmed by a combined 2.5 million since 2008 [source: Wolf]. Explanations for going independent reveal a broad unease with establishment politics, which may have been aggravated in the past 20 years by mounting infighting between Republicans and Democrats, as well as a legacy of negative political campaigning. Likewise, the top two reasons independents offered Pew researchers for not siding with the donkey or elephant: political parties caring more about special interests than citizens' needs and governmental mistrust.

For her 2012 book "The Swing Vote," Linda Killian interviewed a host of self-identified independent voters who echoed similar concerns about the Washington money machine. The 2012 presidential election is tracking to be the costliest campaign in American history, likely racking up more than $1 billion in expenses [source: Salyer]. Snagging a Senate or House of Representative seat isn't cheap, either; Killian cited a Campaign Finance Institute statistic pinning the cost of becoming a U.S. senator at $9 million in 2010 [source: Killian]. In a nation recovering from an economic recession that left scores of households in poor fiscal shape, that kind of spending clearly hasn't sat well with independents.

Independent voters also may decide to distance themselves from Republicans and Democrats because they're caught ideologically somewhere in the middle. A chunk of them espouse socially liberal stances, such as reproductive rights for women, while at the same time preferring fiscal conservatism, including lower taxes [source: Killian]. Demographically, independent voters aren't as easy to categorize. Leading up to the 2012 presidential election, conservative-leaning older men living in rural and suburban areas, and more liberally inclined younger urban single women emerged as two coveted independent sub-groups [source: Penn]. At the same time, Latino and Asian-American voters have increasingly disengaged from partisan politics and now comprise up to 40 percent of independents [source: Ryo]. This handful of independent groups is indicative not only of the challenge of capturing their votes, but also the expanding diversity among U.S. voters.