Here, each campaign with more than 15,000 identified donors is represented by a bubble. The size of the bubble corresponds with the number of known donors and the color with the candidate's party.
The campaigns are arranged horizontally according to the percentage of their donors who live in majority-white zipcodes. Just over 67% of people in the United States live in zipcodes that are more than 50% non-Hispanic white, though people living in these zipcodes make up roughly 75% of donors.
Note that Republican campaigns are grouped more to the right side, indicating that their donors draw more heavily from majority-white zipcodes. While Democratic campaigns draw less frequently from these zipcodes, they also span a wider range. Residents in majority-white zipcode comprise less than two-thirds of donors for some Democrats, and nearly 90% for others.
The gap between parties is more stark when looking, instead, at formal education.
Here, the campaigns have been arranged vertically according to the percentage of donors from zipcodes with high Bachelor’s degree attainment (top 25%). By definition, these make up just 25% of zipcodes, yet they make up virtually the entire donor base for the campaigns near the top of the chart.
Looking at both race and formal education at the same time, provides a clear delineation between Democratic and Republican candidates.
Donors to the Republican campaigns tend to live more frequently in majority-white zipcodes, and live less frequently in zipcodes with high Bachelor’s degree attainment.
For Democratic candidates, it is the opposite. Their donors are often from more racially diverse zipcodes, and zipcodes with high bachelors degree attainment
Note also that Democratic campaigns are generally clustered less closely than the Republican ones.
Look, for example, at the gap between donors to Bernie Sanders' campaign, on the left, and Amy Klobuchar's campaign, to the right, indicating that Klobuchar's campaign drew many more donors from majority-white zipcodes.
The campaigns of Representatives Ocasio-Cortez, Omar, Tlaib, Pressley, and Khanna are also clustered together near the left end of the plot, along with 2020 progressive insurgents Cori Bush and Jamaal Bowman.
A skeptical reader might wonder how much these patterns are simply due to geography. All else even, we would expect donors to Steve Bullock's senate campaign in Montana to look different than the donors to Alexandria Ocasio-Cortez's congressional campaign in the Bronx.
To help account for this, the donors have now been filtered to include only those from outside of a candidate's district or state (for presidential campaigns, nothing has changed). For a few candidates, such as Senators Merkley, Cornyn, and Markey, this makes a big difference. By and large, though, the divides between and within parties look very similar, even with in-district donors excluded.
Finally, the y-axis has been switched to indicate the percentage of donors who come from high-income zipcodes (top 25%). These zipcodes produce a very large percentage of campaign contributions overall (note that the y-axis is clipped at the bottom)
While Democratic donors, on average, are more likely to live in high-income zipcodes, the divide between parties is much more muddled when looking at income level instead of Bachelor’s attainment, particularly among more major candidates. This supports evidence found in polling that party support is divided much more by formal education level than by income. Democratic party candidates, however, span a much wider range, suggesting a Democratic donor’s income level may have a strong impact on the kinds of candidates that they support within the party.
More than 18 million donors from across the country contibuted to candidates running for federal office during the 2020 election cycle, amounting to nearly $7 billion. Online records from the FEC, as well as the ActBlue and WinRed fundraising platforms provide us with more data than ever about where small-dollar donations are coming from.
FEC records don't provide demographic information about individual donors directly, but they do provide a zipcode. And because zipcodes in the United States remain generally segregated, we can attempt to better understand the donors backing various campaigns by looking at the demographics of their respective zipcodes. While gender does not vary significantly by geography, race/ethnicity, education level, and income level often do and these features expose key divides, not just between, but within parties.
Visualization by Sam Learner | 
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Code and data for this project lives here. You can check out a companion project on donor overlap between candidates here.
Notes
Contribution data runs through 10/15/2020.
Unique donors are determined using a combination of first name, last name, and zipcode. This creates the possibility of both false positives (two Jane Smiths in the same zipcode) and false negatives (Jonathan Smith sometimes donates as Jon Smith, or Jon Smith moves homes during the election cycle). Others who have worked with donor data sometimes require a matching employer name as well. This creates a very small chance of a false positive (it's very unlikely that there are two Jane Smiths living in the same zipcode with the same employer), but creates many more false negatives (Jon Smith might change jobs mid-election cycle or he might mark his employer as "Smith High School" on one donation and "Smith H.S." on another). On balance, when looking at donor overlap, I decided that requiring a matching employer was too stringent of a requirement and would lead to missing too many donor overlaps. As it is, the analysis very likely underrepresents the amount of overlap due to false negatives, but it misses them uniformly, across candidates.
The use of the WinRed fundraising platform among Republican candidates is relatively new and still less prevalent than the ActBlue platform among Democratic candidates. Since much of the data regarding smaller-money donors (<$200) comes from the platforms' filings, GOP candidates generally have fewer identified donors than Democratic candidates. Even among candidates who have adopted these platforms, many small transactions take place outside and the campaigns are only required to report contributions from donors who give more at least $200 in aggregate. This means we don't have a true donor count for any campaign, but we can generally get much closer for Democratic ones.
Sources
FEC, ProPublica (for itemized ActBlue, WinRed receipts)
Last Updated: November 2020