Well let's see. In 20 years, those born between 1945 and 1960 will be between and 72 and 87; in other words alot of them will be dead. Most people on the forum today will be anything from 35 to 50odd years old. If the GOP does not shift on social issues it will be dead.
If the GOP dies, then there will be a one party system in the US. That will lead to riots, protests and eventually a 2nd party will form again.
No one's going to argue with that. I can see the TEA party replacing the Republican Party or the Occupy Wall Street replacing the Democratic Party.
In this polarized environment, I would say that if an "in" party keeps total control for 6 years, people will start to question the viability of the "out" party. If it goes on for 8 years, I think at that point many could consider the opposition "dead".
If the Republican Party "dies", the resulting unrest will be a lot like an Eric Rudolph, massive TEA party events with people brandishing guns, OKC or Ruby Ridge happening about every month until a new conservative party takes over at least one branch of the government.
The Democratic analog would be things like race riots in major cities, protests in front of gated communities and conservative churches and the like going on until there is a viable progressive alternative again.