Introduction

American politics has become increasingly polarized over the past five decades. This analysis explores how Democrats and Republicans in Congress have grown further apart ideologically and how this divide manifests in voting behavior. Using data from the 93rd through 119th Congresses (1973-present), we examine party-unity voting patterns, ideological positioning, and coalition-building across different types of legislation. This document contains the first six visualizations, and the attatched Tableau file contains the last two visuals.

Visual 1: Party Overlap Then vs Now

This visualization compares the distribution of member ideology scores in the 93rd Congress (1973-1974) and the 119th Congress (2025-2026) using a common measure that places each member from left to right based on how they actually vote. The ideology score (DW-NOMINATE) was developed by researchers at Voteview, an online database that provides comprehensive statistical data on United States congressional roll-call votes and legislator ideologies. Values below zero indicate more liberal voting patterns, and values above zero indicate more conservative patterns. Data is updated live, meaning the data sourced in this project contains entries as recent as November 2025.

In the early 1970s, the two parties’ curves overlap around the center of the scale. This is consistent with the historical idea that the two major parties were more tolerant of a wider range of ideologies under the same “tent.” A well-known example is the presence of conservative Southern Democrats and liberal New England Republicans in this period, groups whose voting behavior frequently crossed party lines and created meaningful ideological overlap between the parties. By the 119th Congress, the Democratic and Republican distributions have diverged sharply apart. The plot shows almost no overlap, with ideological sorting now taking hold within each party. The magnitude of the two clusters and the low height in the middle shows how the presence of moderates in Congress has thinned out over time.

Visual 2: Moderates vs Extremes in Party-Line Voting

This figure compares how often moderates and ideologically extreme members vote with their party in the most recent Congress (119th). Members are classified based on a threshold on the ideology scale, where those further from the center are classified as “extreme.” Across both major parties a clear pattern can be seen in the boxplots: ideologically extreme members tend to vote with their party at very high rates when compared to their moderate colleagues. Moderates typically support their party, but they exhibit much more variation in their voting patterns. This contrast serves to show the relationship between ideology and party loyalty has grown strong. Members on the ideological edges are the most reliable party-line votes in Congress.

Visual 3: Party-Line Voting by Type of Vote

This visualization shows how often Democrats and Republicans take opposite sides across different types of votes in the 119th Congress. Final passage votes, the decisive vote before a bill is passed, is expectantly highly partisan. Motions and procedural votes are also similarly polarized, showing how control over the legislative agenda is also a significant area of political disagreement. Amendments are modestly less partisan than the other categories, showing how members sometimes cross party lines when voting on specific or technical adjustments within bills. The ultra-partisan “Other” category captures roll-call votes that do not neatly fit into the other three categories, such as quorum calls or specialized procedural actions. Across all categories, the relatively uniform party-line share indicate how far partisanship influences the overwhelming majority of congressional votes in today’s government.

Visual 4: Geographic Polarization by State

This map of the United States shows how far apart each state’s Democratic and Republican congressional delegations are on the ideology scale in the 119th Congress. The darker shading indicates larger gaps in ideology within the state’s delegation. Southern states appear to have the starkest ideological contrasts within their delegations, with the coastal states on each side of the U.S. containing some moderate gaps in their members. Interestingly, most of the landlocked Western states and New England share low ideological gaps. This is likely due to the overwhelming presence of Republican representatives in the Western states and a similarly overwhelming presence of Democratic representatives in the New England states. Thus, the states with low ideological gaps are not “moderate,” but are instead the most polarized areas in on the map.

Visual 5: Shiny App | Coalition Explorer

Shiny applications not supported in static R Markdown documents

This “Coalition Explorer,” built as a shiny app, lets the viewer drill down from broad voting patterns to specific roll call votes. The app allows users to choose a Congress, chamber, type of vote, and coalition pattern (ex. strict party-line or bipartisan) and view a filtered list of votes that match those criteria. The text at the top of the app dynamically shows how many votes are displayed and how many of them were strict party-line. Selecting a specific roll call (or row in the table) yields a bar chart showing the share of Democrats and Republicans who voted yea and nay in that instance. Overall, the app serves a purpose as a way to explore which types of bills are more likely to garner bipartisan support, and which bills are more likely to result in party-line votes.

Visual 6: Shiny App | Polarization Overlays

Shiny applications not supported in static R Markdown documents

This shiny app allows the viewer to compare two Congresses on the same field and see how polarization has changed over time. Users choose “Congress A” and “Congress B,” select chambers and parties, and then see how every member plotted by their ideology score on the x-axis and their percentage of party-line votes on the y-axis. Congresses are differentiated by color, while parties are differentiated by shape. This makes it simple to read trends in the data. For example, a disappearance of moderates or increased clustering near 100% party-line voting is easy to spot. The app also generates a summary table that reports the share of extreme + loyal members by Congress and party.

Conclusion

Overall, the visualizations in this project shows how the ideological and behavioral distance between Democrats and Republicans has only grown over the last five decades. The comparison of the 93rd and 119th Congresses reveals a stark contrast in party unity and the clear collapse of the ideological center. While present in ideological trends, this shift has also manifested itself in how consistently members tend to vote with their party on the floor. Party unity is highest on final-passage votes, though it remains consistently strong across nearly all types of congressional votes. Geography also reinforces these patterns. Many states now elect almost exclusively one party to Congress, and the resulting delegations enhance the national ideological split. The shiny apps show that these trends are not just confined to aggregate patterns. They appear in individual votes, coalitions, and member-level actions across a range of legislative areas. The typical member today is more polarized, more reliably aligned with their party, and more distant from their colleagues across the aisle than in the previous half-century. These visualizations serve to illustrate a somewhat abstract shift into an article that can be viewed and understood by a wider audience. By using the tools of data visualization, we can uncover insights that are valuable for legislative strategy, administrative planning, and understanding partisan behavior.