What does it mean to be a moderate in Congress? One measure might be the willingness of legislators to break from their party's prevailing position on an issue—perhaps driven by regional differences, special interests, or contrarian ideologies to break outside of the left-right partisan divide and spur a multi-dimensional debate.

As Congress has become more polarized, however, policy debates have become less complex, distilled down to a strictly dualistic, partisan choice. "Issue-centric conceptions of congressional voting were replaced by ideological ones emphasizing the power of party and ideology to readily simplify the complexities of congressional life," explain Ashley Joachim and Bryan Jones in a new article for Political Science Quarterly.

Joachim and Jones examined how this trend increased between 1965 and 2004 in the House of Representatives, as legislators became more inclined to vote based along party lines and made the debate over issues increasingly one-dimensional. Rather than become more polarized across the board, Congress began losing alternatives to partisan viewpoints on an issue by issue basis, they explain.

(Source: Political Science Quarterly)

Here's the researchers own take on their findings:

The majority of issues exhibited declines in dimensionality across the two eras, a trend that is perhaps unsurprising given increases in party polarization....a variety of issues (health care, education, civil rights, crime and law, and defense) have evolved from multidimensional space to unidimensional spaces. This suggests that party and ideology have come to intrude into issues once less susceptible to their power.