From 2015: TL;DR- GOP base is more united than the Dem base
The Republican Party is primarily the agent of an ideological movement whose supporters prize doctrinal purity, while the Democratic Party is better understood as a coalition of social groups seeking concrete government action. This asymmetry is reinforced by American public opinion, which favors left-of-center positions on most specific policy issues yet simultaneously shares the general conservative preference for smaller and less active government.
As kwabbit said, the D coalition is more pluralist (racially, ethnically, religiously, socioeconomically, etc. diverse) than the R coalition. Also there are more right-leaning Dem voters than there are left-leaning GOP voters.
https://www.pewresearch.org/politics/interactives/political-typology-comparison-2021/
I also enjoy the (probably correct) observation that more people in the Democratic coalition have essentially Republican orientations than the reverse, so there is still more low-hanging persuasion fruit for the Republicans to pick, on net. (Also, interesting to note that Republicans with Democratic orientations are an overwhelmingly young group -- likelier in every younger age group, and the effect is really strong -- while Democrats with Republican orientations seem to have virtually no demographic characteristics that mark them apart. A little likelier to be Hispanic than a different race, but even that's only a small effect.)
