What the Court said in Citizens United is that when the First Amendment bans Congress from restricting the freedoms of speech and the press, it bans Congress from limiting people’s campaign expenditures. This, it seems to me, is perfectly obvious: if I have freedom to print what I want, I have freedom to pay others to print what I want. They are the same thing.
Socialists want to undo Citizens United because they dislike freedom of the press. Period. In pushing McCain-Feingold, John McCain said that his goal was to insulate politicians from critical advertising too proximate to an election. In other words, his law was as nakedly unconstitutional as it could have been. It is hard for me to understand what people think they are going to gain by abetting politicians who want to be immune to criticism.