Umm ... HUAC was convened by Democrats. They were so-called “progressives” (which was bad enough) but they weren’t yet anti-American socialists.
Before the reagan Democrats there were what we might call the McGovern Republicans, people like Hillary, who were the unwholesome part of the Republican party that swept into the Democrat party to join with the equally vile resident far Left Democrats. They took over the party apparatus. Carter was actually the last real push back agains them to try to dislodge them ... which failed.
Now nothing of the old extreme Left is extreme to them anymore. They probably think McGovern was centerist and LBJ right of center.
That’s kind of a nonsequitur. People like Martin Dies and Reagan were nothing like people such as Wilson (who equated socialism with democracy), FDR, Hiss and their ilk who held power in that party; and RINOs were certainly around back then, e.g. Arthur Larson, who wrote “A Republican Looks At his Party” infamously mimicking Communist sympathizer Dean Acheson (author of “A Democrat Looks At his Party”).