On occasion in American history, concepts like Left and Right become blurred, parties run out of steam and ideas, and a wing of one party wraps around a wing of the other party. Sometimes one party will even splinter. Then the two parties re-form when a new issue arises.
One thing we can all be sure of is that new issues will emerge over time. Economic and social circumstances change, and eventally two sides will form, one favoring stasis and the other favoring change.
After the Cold War ended, Francis Fukayama proclaimed "the end of history." Well, we all know he got a rude shock on 9/11. As did all of us.
Good points. Thanks for your thoughts, Publius.