See here. Bob is essentially concerned that allowing regulation of disclosure would lead to other, more intrusive regulations down the line. The position is a bit ironic. It is the mirror image of the anti-circumvention rationale adopted by the Supreme Court that Bob has criticized repeatedly. That is, the Court says: Congress can regulate local political party activity not because it is shown that local parties have been the site of corrupt federal election activities but rather out of fear that after the national soft money ban local parties could next play that role. On the flip side, Bob doesn’t make arguments against the virtues of disclosure itself but wonders what it would lead to next.
To be clear: in my view there are a great informational benefits for the public to having those paid to make public communications to favor a candidate for federal office disclose that payment as part of the communication itself. And I believe this principle should apply equally both on and off the Internet. I am not advocating greater regulation of bloggers beyond disclosure by bloggers paid by the campaign to write such public communications.