The problem for Democrats is very simple. They expect to win and so are in the process of moving as far left as possible in order to claim a mandate. They will, should they win in November, really not have any mandate for any agenda other than being "not Trump." But they want to claim it.
That far left drift is a problem. Black and hispanic voters currently have no other party to go to, but there is ample evidence they are starting to grow concerned by the radical secularism invading the Democratic Party that makes it increasingly hostile to religion. Blue collar white voters, likewise, are less and less enamored with the Democrats.
It is becoming a party of secular atheists and rich white people. It seems its only saving grace is a GOP hell bent on self-destruction.
I suspect the Democrats will see their wave roll through in November. The problem, though, is that in their dramatic lurch left they undercut some of their wave. They may see some seats stay Republican that a more moderate party could have taken. On top of that, they will no doubt interpret their win as a mandate and, in lurching far left, set themselves up for failure in 2020 even against a more restrained President Trump.