I'm generally for more disclosure in campaign finance, but the best argument against requiring full disclosure by groups engaged in political speech is that politicians sometimes retaliate against their critics. Sen. Elizabeth Warren inadvertently made that very argument this week.
As told by Ben White at Politico, a group called "Third Way" criticized Warren. Warren apparently suspected that Third Way's criticism of her was funded by banks. So she wrote a letter to bank CEOs demanding they disclose which political groups they're funding.
Warren sits on the Senate Committee on Banking, Housing, and Urban Affairs. She's basically telling the entities whose livelihood her committee controls to stop criticizing her. This is bullying — and it's the best argument for allowing companies and individuals to anonymously criticize politicians.