You can manufacture a favorable electorate. Republicans have been extensively working on that far harder than the Dems have since some time around Goldwater and the last great re-alignment, and it kicked into overdrive in the 80s. They pushed for loosening rules around mass media so they could do it better, and they succeeded. This current re-alignment of their party is an outcome of that “farming” they did over decades growing out of control of the party leadership post-Citizens United and the huge shake-up in campaign spending that brought in.

This observation admittedly provides little actionable for democrats in the near-term. But one strategy that demonstrably works is picking demographics and pushing media at them that creates a demand for solutions to issues they didn’t previously think existed (and need not necessarily exist). Look at e.g. the molding and elevation of the modern pro-life movement for an early example, or at their entire current platform, very nearly, for a bunch more-recent ones.