If there ever is a dominant "one party" in the country, it will be politically quite middle of the road, where the majority of the country's voters sanely lie.
Sadly, the Democrats are progressively too liberal in their leadership and platform, and faithfully incompetent when in power. The GOP has been co-opted by mostly aging white, rural and sanctimonious nut jobs who can't separate the church from the state.
Fortunately, there is enough Independents out their who currently don't adhere to either party ad can swing the elections towards the most middle of the road politicians who, whether DEM or GOP aren't swayed by partisan claptrap and impractical dogma. In the end, we win.