CPRC Dataset (94 Mass Public Shooters, 1998–2021)
This conservative-leaning research center's analysis found:
No identifiable political leanings: 73%
Islamic extremist leanings: 8% (e.g., cases like the 2015 San Bernardino shooters)
Right-wing or conservative leanings: ~5% (primarily tied to white supremacist or anti-immigrant views, e.g., Dylann Roof in 2015 Charleston or Patrick Crusius in 2019 El Paso)
Left-wing or liberal leanings: ~5% (e.g., environmental extremism or anti-police views, such as the 2016 Dallas shooter Micah Johnson)
Other/known but mixed: ~9% (not broken down further, but includes rare cases with vague or conflicting affiliations)
PolitiFact Analysis (19 High-Fatality Mass Shootings, 1999–2018)
This fact-checking outlet reviewed cases with 8+ fatalities and found even higher rates of unknown leanings:
No identifiable political leanings: 89%
Democrat-leaning: 5% (e.g., Omar Mateen, 2016 Orlando shooter, registered Democrat)
Republican-leaning: 5% (e.g., Chris Harper Mercer, 2015 Oregon shooter, self-identified Republican on a profile)
Liberal-leaning: 5% (e.g., Aaron Alexis, 2013 Washington Navy Yard shooter, described as a "liberal type" by a friend)
Other: 0% (speculative claims for several others were debunked)
I'm counting Islamic as left since they are allies of the left.
