Tax-exempt “social-welfare” organizations have become the main conduit for large campaign contributions, ProPublica reports as part of a broader examination of such groups’ activities and their potential influence on the 2012 election.

The nonprofit news outlet reviewed Internal Revenue Service and Federal Election Commission filings for more than 100 organizations that have or are seeking 501(c)(4) status and were active in the 2010 election cycle.

Some groups that said in applying for tax-exempt status that they would not engage in politics have since reported spending millions of dollars on campaign ads, many praising or criticizing particular candidates, and some appear to spend money on little else, ProPublica writes.

Under IRS regulations, 501(c)(4) entities are not allowed to engage “primarily” in politics. As of Aug. 8 such groups, which are not required to name their donors, had spent more than $71 million on ads mentioning a presidential candidate, compared to $56-million from super PACS, according to estimates by Kantar Media’s Campaign Media Analysis Group.