1st, the DemonRats will become more and more marginalized and a successful 3rd party will arise. That's only happened twice, I think, and never in this century (emphasis on "successful" meaning they become stronger and stronger and overtake one of the primary parties). This can occur if the blue-collar (non-union) democrats, the fiscal conservatives, the "Goldwater" Republicans, and some others create a 3rd party that will absorb the non-leftists from the Democrat party as well as the fiscal conservative/Goldwater Republicans. I make no likelyhood guess as to how long this would take, but I would guess it would depend on how long it takes for a dominant Republican party to become corrupted.
Alternatively, I could see the Democrat party have a revolution and kick out the Leftist demon possessed gay/green/pediphile wing. If they did that successfully and became the party of fiscal conservatism, strong border control (anti-illegal immigration), and strong military defense they could then challenge a dominant Republican party. But the DemonRats would have to be absolutely trampled into the mud for many years before this could happen. Still -- 20 years -- maybe.
The bottomline: As long as the only issue that the Democrats have to offer is the right to kill one's unborn child, they are doomed to minority status. I'll cheer the day they sink into the abyss. That's objective #1. When that happens, we can then debate the future of the Republican Party.