Read this exchange between the Senator and a “Think Progress” blogger. I think the blogger is focusing only on the disclosure provisions of the DISCLOSE Act, which appear to me (at least on the surface) to treat corporations and unions equally. But Senator McCain is likely focusing on other provisions, such as those which limit certain corporate spending (of government contractors/TARP recipients/certain American subsidiaries of foreign corporations). Some opponents of DISCLOSE have argued that some of these provisions could be extended to certain union entities as well. This is likely what Sen. McCain was thinking about as he answered the blogger’s question.
But it does show that McCain is not a likely vote to bring DISCLOSE to the Senate bill. It is hard to see a path to the Senate floor right now.