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.

Officials put no timeline on the process, disappointing those who had hoped the new regulations might kick in before this year’s mid-term elections.

“I think it’s unfortunate that new rules will be delayed even further and that we’re going through another election cycle” without them, said Paul S. Ryan, senior counsel with the Campaign Legal Center.

Others called the delay a prudent step that would give the IRS an opportunity to get a crucial change right.

“They’re not going to put out some slapdash rule just to check it off their list,” said John Pomeranz, a Washington lawyer who works with nonprofits that spend money on politics. He doesn’t expect the agency to finish the rules any time soon. “I think we’ll be lucky if they’re in place for the 2016 election.”

Social welfare nonprofits have poured money into politics since the Supreme Court’s Citizens United decision in 2010, which allowed corporations, unions and nonprofits to spend unlimited money on elections.

Social welfare nonprofits spent more than $256 million in the 2012 cycle alone, according to the Center for Responsive Politics. Campaign finance watchdogs have viewed their rise with concern, fearing the influence of so-called “dark” money from secret donors, and had called for more oversight from the IRS.