The amazing thing to me about the Democrats in recent decades is that if they had just remained a centrist party they would probably have an unbeatable majority, either already have it or soon have it. The demographics are on their side. The Republicans can do well with Hispanics, but the Democrats will always have an edge with that fast-growing group.
So what did the Democrats do at the same time they were accruing this growing demographic advantage? They went insane. They adopted the the politics of treason and socialism. Like their friends the Palestinians, they are missing opportunities of historic proportions.
I'm not sure they 'went' insane. The party was taken over by the far left fringes. Those of moderate temperament who remain have little say so in the party's direction.
The Democratic Party has been anti-war from virtually its inception. Even during the Civil War the Democrats preferred allowing the Southern states to secede rather than fighting to keep the Union whole. They even (at the time) were willing to allow slavery to continue. This makes the popularity of the Democratic Party among Blacks so very ironic.
I believe the seeds for the party's irrelevance had been sown a long time ago. What they've become is just a natural progression from what they've been all along.