McCain et al. also say that if the ban on soft money is struck down, they "will face the strong risk that unregulated soft money contributions will again be used in an attempt to influence federal elections in which [we] are among the principal participants." Note two things.Since passage of the Tillman Act in 1907, it has been illegal for corporations to contribute to campaigns of federal candidates, and in 1947 the Taft-Hartley Act brought labor unions under the same restrictions.
Furthermore, McCain et al., in this assertion that they would be harmed by overturning BCRA's new regulations on campaigning, make no mention of the only thing the Supreme Court says justifies such regulations on political speech: preventing corruption or the appearance thereof.
They make no mention of it because corruption is not what they have uppermost on their minds. What is uppermost is in McCain et al.'s wonderfully revealing description of how overturning BCRA would harm them. They say that if the restrictions on advertising within 30 days of primaries and 60 days of general elections are struck down, they "will face attack in broadcast advertising campaigns mounted by corporations and labor unions."
Imagine that. The poor dears.