Any argument that's based on alliances in a two-sided power struggle is not going to be informative about the actual beliefs of the allies. By that sort of reasoning, you could prove that not only Roosevelt but Churchill was a Communist—given who they allied with in WWII. Or you could argue, as a conservative former friend did to me, that since I am opposed to making Christianity the American state religion I must be a support of Muslim conquest of the United States and replacement of the Constitution with sharia.

In-group/out-group thinking, of which left/right thinking is one of the currently popular forms, does not support any sort of rational analysis. And it tempts people into forming precisely the sort of fatal alliances you describe in Spain.