The United States has only lived up to basic standards of democracy for a few decades of its history. In parts of the South, the last scraps of Jim Crow tyranny were only rooted out through massive federal coercion in the mid-1970s. That said, there has almost always been genuine competition for national power between the two parties — the one exception being the Civil War, but even in that case the South attempted to secede from the country rather than conquer the whole thing.
That competition may come to an end very soon. Democrats right now control the presidency and Congress by a very thin margin — just one vote in the Senate.