After intense criticism from both ends of the political spectrum, the Internal Revenue Service has delayed indefinitely proposed rules that would have imposed new limits on social welfare nonprofits, which have pumped hundreds of millions of dollars from anonymous donors into recent elections.

The agency said yesterday it would postpone a hearing on the proposal it released in November defining more clearly what constitutes political activity for such groups, and would revise the plan to reflect some of the more than 150,000 comments it triggered.