I couldn't find data on volunteer rates in the USA, but according to this article about the UK, LibDems & Conservatives volunteer more than Labor: https://medium.com/@sheffeconomics/do-your-political-views-make-you-charitable-4c7057d1c4d - Greens, on the other hand, volunteer the most, but there're so few of them compared to the other 3 parties that it's pretty clear that the overall average would be conservatives volunteering more than progressives/leftists.