Visualizing the US congress

Published Nov 2016

Last updated Aug 2017

This is a interactive visualization I made of the congress members’ ideology positions, reduced to 2 dimensions, using the DW-NOMINATE method. It is developed as part of the IDEAS Focus Summer School on Data Visualization at Northwestern University.

How to use this visulaization

Visualizing the US congress

congress # 113

year 2013 - 2015

What are the axes?

The DW-NOMINATE 1st dimension (horizontal axis) can be interpreted as the position on government intervention in the economy, also referred to as the ‘liberal-conservative’ scale. The 2nd dimension (vertical axis) can be interpreted as the position on salient issues of the day, such as slavery, civil rights, and LGBT rights. The 1st dimension explains the vast majority of differences in voting behavior.
(according to Poole and Rosenthal, more discussion here).

How are the positions calculated?

The DW-NOMINATE method gives Congress member’s relative ideological positions through analyzing how often Congress members vote with or against each other in the roll call records. The DW-NOMINATE method is a scaling method – it puts those with more similar voting behavior closer together, and vice versa. Its Wikipedia page gives a good explanation of the method.

More information

The U.S. Congress members’ roll call vote records are openly available. You can explore the data set and download it from More information about the DW-NOMINATE method and data download can be found on the The original page where this data is downloaded is depreciated. You can find the csv file used in this visualization here. The updated voteview page for data download is here. This interactive visualization is developed using D3. If you are interested in making similar visualizations, you are welcome to download the source code of this page on my Github.