For the first time since 1959, American nonprofit advocacy groups face new Internal Revenue Service rules governing their political activities, an area of the tax code that has been crying out for greater clarity.

A proposed regulation unveiled Tuesday by the Treasury Department draws the boundaries more clearly — but kicked off intense debate about whether the lines are in the right place.

One phrase in the official notice summed up the imperfect nature of the exercise. The new rules, the department said, "may be both more restrictive and more permissive than the current approach."