Ray La Raja is one of the leading political scientists who writes on campaign finance issues and political polarization. With his co-author, Zach Albert, he has a new working paper that examines small-donor funding patterns for moderate and extreme candidates from each party.
Their key finding on this issue: in 2018, the most extreme Democrats received 86% of their funding from small donors, while moderates received only 10%. On the Republican side, more extreme candidates received 58% of their funds from small donors while moderates received only 17% of their funding from small donors.
The abstract is here, and the working paper can be downloaded in full from the American Political Science Association website at that link, for those who have access.