This viewpoint is often touted by those on the political left but has no evidentiary basis that I've seen, certainly. At most the democratic party has a strong tendency of allowing its most fringe members an out-sized voice in the interest of inclusion, which only pushes away its much larger moderate base. Perhaps the form of "compromise" you are referring to is just the more extreme viewpoints being filtered out as actual reasonable policy proposals are formed.

EDIT: Which, by the way, also happens in the Republican party. See, for instance, the repeal vs replace movement and how those went nowhere.