I can’t help but recall that, in the last ten years, we’ve been confidently informed by partisans and pundits of both the “permanent Republcan majority” and the “new Democratic majority” and the supposed relegation of the G.O.P. to a “rump party.” Both claims have been falsified in pretty short periods of time. Can we now accept that what we have in the U.S. is a more-or-less permanent see-sawing between the conservative party and the liberal party and that neither one is ever going to be able to claim decisive once-and-for-all victory? As T. S. Eliot memorably put it, “there is no such thing as a Lost Cause, because there is no such thing as a Gained Cause.”
A Thinking Reed
"Man is but a reed, the most feeble thing in nature, but he is a thinking reed" – Blaise Pascal
A Thinking Reed 
There are a variety of structural, political sciencey reasons–such as our winner-take-all elections process, or the fact that gerrymadering has become an exact science placed in the hands of incumbent state legislators every 10 years–why America comes out so deeply divided, and so capable of wide swings one way or another, when elections role around. But that shouldn’t completely invalidate the arguments made by all those partisans and pundits; their conclusions may be premature or inaccurate, but the data they are looking at–whether it be the rise of unmarried college-educated working women for the Democrats, or the Catholic Hispanic vote for the Republicans–isn’t going away. Parties get deeply entrenched, thanks to the power of money and interests groups over the process, but ultimately all of them have to have a dependable coalition of voters. When those coalitions fall apart, genuine re-alignments are inevitable. Whether such a re-alignment coincides with any single electoral victory is a different question though, of course.