WASHINGTON — In early 2010, a new organization called the Commission on Hope, Growth and Opportunity filed for nonprofit, tax-exempt status, telling the Internal Revenue Service it was not going to spend any money on campaigns.

Weeks later, tax-exempt status in hand as well as a single $4 million donation from an anonymous benefactor, the group kicked off a multimillion-dollar campaign against 11 Democratic candidates, declining to report any of its political spending to the Federal Election Commission, maintaining to the I.R.S. that it did not do any political spending at all, and failing to register as a political committee required to disclose the names of its donors. Then, faced with multiple election commission and I.R.S. complaints, the group went out of business.

To Democrats and some campaign finance watchdogs eager to force more transparency in spending by independent groups, Hope, Growth and Opportunity stands out as the kind of “pop-up” organization that operates in virtual secrecy, with legal impunity.

“I still don’t know who they are,” said John Spratt, the former chairman of the House Budget Committee, who lost his 2010 re-election bid after facing a deluge from the group. “It’s a classic case.”