Mr. Berman said his center hoped to help enact a Republican-backed bill that would prohibit unions from organizing workers through card checks.

Following publication of the study a state statute was adopted prohibiting unions from making the selection of health insurance providers a bargaining issue.

The groups began playing a prominent role last year after new campaign finance laws prohibited major donors, unions and corporations from giving unlimited amounts of money to parties.

The chief Republican proposal carries a second sentence that some interpret as prohibiting civil unions, as well.

It would prohibit unions from using payroll-deducted funds for political purposes (the same restriction applying to payroll deductions, if any, by corporations and government contractors).