A trend goes beyond just American politics, and can be seen as repeatable over time. Namely, from the 1800's when Communists saw Social Democrats as collaborators and generally allowed Conservatism to dominate the first half of the century, and then allowed conservative Emperors to adopt some socialist policies in their effort to advance nationalist empires(see: Bismark and the German Empire, Napoleon III, Victoria). Or Germany in the 1930's when there were more Communists and Social Democrats than Fascists, but their inability to come together allowed the Nazi's to take over with 33% of the vote.
Or the fact that, in America, there are far more consecutive conservative administrations than progressive ones. I might be wrong, but I think the only time the progressive party held the White House through two consecutive presidents are those times when the first died in office (FDR, Kennedy).
And, it's about policy. Like I said, it's deeper than American partisanship.
The fact is, progressives and conservatives are still rooted in classical liberalism, where socialism is antithetical to both interpretations of liberalism.
This is ancient history now.