Ray La Raja is one of the leading political scientists writing on issues of campaign finance and political polarization. Along with his co-author, Zach Albert, he has a new working paper that looks at the small donor funding models for moderate and extreme candidates from each side.
Their main findings on the matter: in 2018, the most extreme Democrats received 86% of their funding from small donors, while moderates received only 10%. On the Republican side, the more extreme candidates get 58% of their money from small donors while moderates get only 17% of their money from small donors.
Summary is here, and working documents may be downloaded in full from the American Political Science Association website at that link, for those with access.