Party identification among Colorado residents, 2006-2025

Over the past 20 years, Democrats have gained a substantial advantage among the general public in Colorado. This is in stark contrast to the way Democratic identification has been trending in the rest of the country. Two decades ago, the share of Coloradans who identified or leaned toward the Republican Party was about the same as the share who identified or leaned toward the Democratic Party. Today, Democrats hold a clear majority in party identification in the state.

Share of Coloradans who are ...

Republican identification in the general public declined substantially during the first Trump administration. This was accompanied by a large increase in the share of Coloradans who say they don't have any partisan leaning. There is some indication that those trends started to reverse during the Biden administration with a slight uptick in the share of Coloradans who identify or lean toward the Republican Party and a concomitant decrease in the share who say they have no affiliation.

What we talk about when we talk about party identification

Partisan identification – an individual's attachment to a political party – is the most important individual-level factor in contemporary U.S. politics. When political scientists talk about party identification, we are talking about something different than partisan registration (officially registering as a Democrat, Republican, Libertarian, etc.).

Partisan registration is an administrative fact that is used to control access to primary elections and provide party organizations with lists of potential supporters. For individual voters, the primary consideration when it comes to partisan registration is some combination of their own political identity, the competitiveness of the districts they find themselves in (in many places in the country, the only meaningful election is the primary contest) and the rules governing participation in primary elections (some places require voters to be registered with the party that is holding the primary contest – or in the case of Colorado, at least not be registered with an opposing party).

Party identification is a psychological or sociological attachment to one of the parties. People identify with parties for many reasons, and though it may sound redundant to say that those who identify as Democrats vote for Democrats and those who identify as Republicans vote for Republicans, political scientists have identified a range of seemingly independent effects of partisanship on individual behavior and attitudes. Importantly, partisanship is a stable individual orientation. In surveys where the same people are interviewed repeatedly and multiple points in time, it is very unusual to see dramatic changes in their partisanship.

The data used in this post

The Cooperative Election Study (CES) is one of the largest continuously running surveys of the American public. It has been fielded every year since 2006, and each wave includes interviews with tens of thousands of Americans. Over the 20 years of the CES's history, it has included interviews with more than 12,000 Coloradans. Even with so many interviews, the CES wasn't really designed for state-level analysis, and especially when we look at subgroups within the Colorado public, the sample sizes get very small.

The Current Population Survey is a very large government survey that is designed for state-level analysis. However, it does not include measures of partisanship. We can, however, leverage the large sample size of the CPS with the quantities of interest in the CES. Essentially, I used the CES to get estimates of partisanship among different groups within the public to estimate the partisan trends with each group and projected those estimates onto the CPS population estimates. For more on this and related approaches, see this.

For the analysis reported here, I used a total of 24 groups. Groups were defined by birth year (3 levels: people born after 1980, people born between 1980 and 1959, and people born in 1960 or earlier), education level (no college degree, 4-year degree or more), gender (male, female) and race/ethnicity (non-Hispanic White, other races or people of Hispanic descent). The key assumption of the model is that trends within these cohorts are relatively smooth (an assumption backed up by a great deal of political science research on the topic).

One last definition before we get into some more results. Party identification in the CES (and almost all high-quality surveys), is measured with a series of two questions. Respondents are first asked if they identify as Democrats, Republicans or Independents. Those who do not identify with one of the major parties are then asked if they lean toward one of them. In most samples, something between 80-90% of people will say they either identify or lean toward one of the major parties, and a lot of political science research suggests that there isn't actually much material difference between people who say they identify with a party and those who say they lean toward that party. For everything that I am doing here, I will be defining partisans as those who either identify or lean toward one of the major parties.

Birth cohort

Partisanship has been shown to be impacted by political events in people's formative years (see, for example, this very cool interactive from the NYT Upshot). Even with the relatively large sample sizes afforded by the CES, I don't have enough cases to look at fine-grained definitions of birth cohorts. For my purposes, a coarser grouping will have to do. Here we will look at three groups of people defined by their birth years.

In 2006, only 13% of Colorado adults were born after 1980 (a grouping that corresponds to Gen Z and Millennials). Today, they make up about half of all adults in Colorado. On the other hand, 20 years ago, people born in 1960 or early made up about half of all adults (a grouping that includes the bulk of the Baby Boom generation as well as the so-called Silent generation and older). Today, this group accounts for only one-in-five adult Coloradans. The middle grouping (which includes Gen X and the youngest set of Baby Boomers) has dropped from about 40% of the adult population in 2006 to just about 30% today.

Partisan balance by birth cohort, 2006-2025

When looking at the overall balance of partisanship, the generational gap in Colorado has mostly closed. Today, all three groups are about 20 percentage points more Democratic than they are Republican. During most of the period under consideration, there were significant generational gaps in partisan balance.

For most of the early half of the time series, adults who were born in 1960 or earlier were more Republican than they were Democratic. Around 2016, Republicans had a 10 percentage point advantage with this group. During the mid-2000s and 2010s, there were clear differences between Coloradans based on their age. The oldest were most Republican, and the youngest were much more likely to be Democratic in their affiliation.

All generational groups in Colorado began to move in a Democratic direction beginning around 2016. The oldest group showed the most rapid movement. That trajectory continued for two older groupings, but beginning in about 2020, the youngest grouping of Colorado voters began to move toward the Republicans resulting in the convergence we see today: the oldest group has basically caught up to the middle group, while the youngest group of Coloradans has shifted in the opposite direction.

Concluding thoughts

This is the first post in a brief series on the dynamics of partisan identification in Colorado. The next posts in the series will focus on other demographic factors (gender, education, race/ethnicity) and their relationship to partisanship in Colorado.

The trends discussed here by birth cohort showed a surprising pattern. While much of the past 20 years has been characterized by large partisan gaps between the different generational groups in Colorado, those gaps have largely disappeared. It is too early to tell if this marks a period of transition where a different constellation of partisan alignments by generation might emerge.

It is tempting to project out the generational trends here and speculate about a possible future where the youngest generation is less Democratic than its older counterparts. There has been a great deal of discussion about the political changes among younger people – particularly younger men. There are some indications that these shifts have been short-lived. In Colorado, at least, the youngest part of the public remains solidly Democratic overall (declining from a 2020 high point of D+30 to just more than D+20 today).