Several books by Lefties have made the point that demographics are on the side of the Left. They argue that minorities are growing and are increasingly Democrats. They argue that young people especially single women are increasing in numbers and are decidedly Democrats. Lastly they argue that there is a migration back to central urban cores and those are Democratic population centers.
They argue that the reason they do not control the House, which is pretty representative of changing populations is that many of the districts are gerrymandered in favor of Republicans.
I pay attention to data and this would be very disturbing if it were true. I was helped in my thinking by this article in Commentary.
The article points out that second generation minorities and immigrants do not vote decidedly Democrat. So time is not moving in the lefty direction. Importantly young people seem to change their opinions on a regular basis, cyclically. Sometimes they are more Democrat sometimes they are less.
Single women are a very different matter. They are offset by increasingly hostile males who are voting against whatever position single women take. Males constitute a large and growing population. They and their wives are operating as a counterweight to single women.
Lastly and most importantly the demographic problem of population moving to urban areas is actually detrimental to Democrats because there is virtually no gerrymandering of any consequence. What is happening is that when you go from a 50% Democrat congressional district to a 90% Democratic congressional district, like the one I live in, you still have only one representative. The district you left, when you moved, now has a better chance for the Republicans to win. Making urban districts more Democrat is counterproductive from both the point of view of the House and the Senate.
The demographics are not in fact beneficial to the Left.