Party loyalty trumps independent thought
As reported in the January 27, 2014, edition of the Washington Post, Thomas Carsey, a professor of political science at the University of North Carolina at Chapel Hill, and Geoffrey Layman, professor of political science at the University of Notre Dame, blamed "identification with and commitment to political parties" as the primary cause of what they label "conflict extension." The researchers define conflict extension as "the growth of party polarization across all of the major issue agendas in domestic politics."
The researchers claim that because most people "feel an emotional connection to and social identity with their political party," they tend to change their political positions to match those of the party they identify with. As each major party moves to the more extreme end of the political spectrum on nearly every issue, the party members move along with them. As an example, the researchers cite support for NSA surveillance programs: in 2006, 37 percent of Democrat identifiers and 75 percent of Republican identifiers approved of the surveillance. By 2013 those numbers had reversed: 64 percent of Democrat identifiers approved, and 52 percent of Republican identifiers approved. Rather than indicating that massive numbers of people had switched parties, the change is attributed by the researchers to the fact that there was a Republican in the White House in 2006 and a Democrat in residence in 2013.
As Carsey and Layman conclude:
"A large part of why Democrats and Republicans hold increasingly divergent issue positions is simply because they are Democrats and Republicans committed to their parties. Parties signal to voters where they stand on issues, and those who identify with and are committed to a party view those signals through partisan colored lenses and adopt the positions of their partisan 'teams.'”
It follows that the party leaders are in control, and they stake their claim to political positions in an effort to cement their control. The Supreme Court's decision in McCutcheon v. FEC may have cemented party leader's control of the U.S. political system. As Lisa Mascaro points out in the April 3, 2014, Los Angeles Times, "the demise of the $123,200 limit for the two-year election cycle [allows] party stalwarts such as House Speaker John A. Boehner (R-Ohio) and Senate Majority Leader Harry Reid (D-Nev.)... to raise multimillion-dollar checks from wealthy contributors for new campaign committees."
Mascaro states that following the 2010 Supreme Court decision in Citizens United v. FEC, "corporations, unions and the very wealthy [were freed] to spend unlimited sums on independent election campaigns." This allowed "outsiders" such as "Karl Rove's American Crossroads and the Democratic-aligned Emily's List" to usurp some of the political power wielded by the Democratic National Committee, the Republican National Committee, and their counterparts in the House and Senate, according to Mascaro.
Justice Stephen G. Breyer's dissenting opinion in McCutcheon warned of the potential corruption that could result from the decision:
"Will party officials and candidates solicit these large contributions from wealthy donors? Absolutely. Such contributions will help increase the party’s power, as well as the candidate’s standing among his colleagues.
"Will elected officials be particularly grateful to the large donor, feeling obliged to provide him special access and influence, and perhaps even a quid pro quo legislative favor? That is what we have previously believed." Breyer cites McConnell v. FEC: “Large soft-money donations at a candidate’s or officeholder’s behest give rise to all of the same corruption concerns posed by contributions made directly to the candidate or officeholder.”
In his majority opinion in McConnell, Chief Justice John G. Roberts Jr. countered Justice Breyer's concerns by inviting Congress to pass legislation prohibiting "illegal earmarking" of large campaign contributions to any single candidate. Justice Roberts seems not to appreciate the irony of expecting members of Congress to enact a law that goes against not only their individual interests, but also those of the party to which they owe their elected position.
 McCutcheon v. FEC (2014) 134 S.Ct. 1434 [188 L.Ed.2d 468].
 Citizens United v. FEC (2010) 558 U.S. 310 [130 S.Ct. 876, 175 L.Ed.2d 753].
 McCutcheon v. FEC, supra, 134 S.Ct. 1434 at 1472-73.
 McConnell v. FEC (2003) 540 U.S. 93, 308 [124 S. Ct. 619 | 157 L. Ed. 2d 491].