Super PACs have become dominant forces in the GOP presidential nominating process, sponsoring millions of dollars’ of TV ads in places like Florida. Now a Democratic lawmaker is taking the first steps toward reining them in, at least indirectly, via the tax system.

A little-known provision of the tax law appears to impose a gift tax on large contributions to 501(c)(4) social-welfare organizations, which often serve as conduits for donations to Super PAC’s. (Tax-exempt social-welfare organizations are popular for this purpose, because they can generally avoid disclosing the identities of their donors.) But the IRS has been slow to enforce the gift-tax provision to 501(c)(4)s, in part because of ambiguity in the law and years of inaction by lawmakers and bureaucrats alike.

Sen. Maria Cantwell (D., Wash.) was expected to file an amendment to a transportation tax measure this week that would do two things. It would clear up the ambiguity, saying that “under present law, gift tax applies to transfers to… 501(c)(4) organizations.” It also would require organizations receiving large contributions to notify donors of their potential gift tax liability. Organizations that fail to provide the notice would be liable for a penalty of 50% of each reportable transfer.

The Cantwell provision doesn’t appear likely to pass anytime soon. But it is likely the opening shot in a much bigger conflict over campaign finance…