“We’ve complained about a few big fish and we’ve heard nothing from the I.R.S.,” said Paul S. Ryan, senior counsel at the Campaign Legal Center, which filed many of the complaints with the agency. “We would far rather see scrutiny of these big fish — the groups that spent hundreds of millions of dollars to influence elections — than to see the resources spent on hundreds of small groups that appeared to spend very little on elections.”

Almost all of the groups in question are organized under 501(c)(4) of the tax code, which regulates nonprofit groups engaged in promoting “social welfare.” At least 16 such organizations spent a million dollars or more on campaign advertising in the 2012 elections. (Crossroads GPS and Priorities USA also run “super PACs” with similar names that spent millions more.)

But because they purport to be engaged primarily in issue advocacy, not election advocacy, tax-exempt groups are not closely regulated by the Federal Election Commission. That task falls, instead, to the I.R.S., which can take years to investigate problems and is required to do so in strict secrecy.

Some groups, like Crossroads, filed applications for I.R.S. tax-exempt status, claiming that they would be engaged primarily on research and educational activities but spending the bulk of their money on what appears to be political advertising. Others, like the American Tradition Partnership, operate for years at a stretch without filing federal tax returns, in seeming violation of the law. Many boast of their impact on political campaigns.

But while a few of the big groups have faced delays in having their tax exemptions recognized by the I.R.S., none appear to have received the intense scrutiny given the Tea Party groups, which were asked dozens of questions about spending on political advertising and other election activities. Of 15 such groups represented by Jay Sekulow, a lawyer with the American Center for Law and Justice, none spent a dollar on broadcast advertising from 2009 through 2012, according to the Campaign Media Analysis Group.