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The Obama administration on Tuesday moved to rein in the “dark money” groups that have spent millions of dollars in political campaigns without disclosing their donors.

In draft guidance to be issued by Treasury and the IRS, the administration proposes new standards for tax-exempt groups like the Karl Rove-linked Crossroads GPS and Organizing for Action, the offshoot of President Obama’s own campaign apparatus.

“This proposed guidance is a first critical step toward creating clear-cut definitions of political activity by tax-exempt social welfare organizations,” said Mark Mazur, Treasury’s assistant secretary for tax policy.