Politically active nonprofit groups such as Crossroads GPS and the League of Conservation Voters could see their influence on campaigns dramatically curbed under a proposed rule announced Tuesday by the Treasury Department and Internal Revenue Service.

The new guidance would amend current regulations governing groups set up as “social welfare” organizations under Section 501(c)4 of the tax code. Such groups are supposed to make social welfare their "primary" purpose under current IRS regulations, a rule that is generally interpreted to mean that 501(c)4s must spend at least 51 percent of their funds on things such as community beautification and education.