The Democrats have gained a ton of new voters from the rise of the nones. They have also lost a ton of voters with the defection of millions of White Christians. (Whether this is a function of vote switching or generational replacement is a debate for a different time.)
...All this brings me back to what I believe is one of the most important papers I’ve ever read in political science: Activists and Partisan Realignment in the United States by Miller and Schofield. Here’s their theory. Every time a party (the Democrats, in this case) tries to appeal to a new set of voters (nones) it leaves the other part of its flank exposed (white Christians). The opposite party then swoops in and takes over that part of the electorate.
Thus, parties continue to try and make their tent bigger, which inevitably pushes other folks out of the tent to be scooped up by the opposing party.