Disclosure makes threats possible, and fears of retribution plausible. Within weeks of a contribution of $200 or more, the contributor’s name appears on the public record. Contributors know this, and they know that supporting the challenger can, should the challenger lose, have consequences in terms of future attention to their interests. Of course no incumbent will admit to issuing threats or seeking retribution, but the perception that both exist is widespread.
Odd, I always thought making a major donation to a political challenger, with full public disclosure, was an open threat by the corporate donor to the incumbent. Besides, it's not like big donations like that aren't played up by both the recipient and the opponent. Joyner's analysis is also odd:
There’s no way to simultaneously inform voters about who’s backing a candidate and keep that information secret from other candidate, including the incumbent. And it’s quite plausible, indeed, that victorious incumbents will hold grudges against those who spent money trying to get them ousted.
Is that rational concern enough to justify making political donations secret? After all, the secret ballot–the notion that it’s nobody’s business who you’re voting for–is a cherished part of of political culture and publishing one’s donations pulls the veil off.
If you're a corporation making a major donation to a candidate or an industry group as a publicly-traded company, you owe it to publicly disclose that to your stockholders at the very least. But you can't have a system where you say "donations are politically protected free speech" and then say that the people cannot be made aware of the "speech" happening. That's ludicrous, and it exposes the Citizens United decision for what it is: a benefit to allow more corporate control of our political system.
On the contrary, the fewer limits we put on donations, the more stringent disclosure rules we need. If you're going to make the business decision as a corporate donor to back Candidate X in a public election, your employees, your customers, your business rivals and Candidate X's opponent all deserve to know.
Of course the real solution is to bar corporate donations completely, since corporations don't vote at all, but that would make too much sense.