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.)
