In two words: single women. Put simply, there are more women than men--there always have been--and there are now more single women in the United States then their have ever been. What does the modern Republican Party have to offer them?
No reproductive rights. In fact, no access to family planning and related health services at all. No increase in the minimum wage. No paid family leave. No rights in the workplace whatsoever. No protections against gender discrimination in any form. No financial security in their retirement years. And, increasingly, not even the ability to vote.
The Republican Party's message to women is this: "Sorry, honey, you're not the demographic we're interested in chasing. At least not politically, heh heh heh." No, they're interested in chasing a much different demographic--one that is dying, even as we speak.
The future belongs to Democrats because the future belongs to women. That's where "a woman's place" truly is. The Republican Party will discover that or die. I'm fine with either outcome.