Hatch, in a letter Tuesday to IRS Commissioner Douglas Shulman, said, "evidence suggests that the IRS may have been the source of the unauthorized disclosure of donor information."
The Republican Utah senator was referring to the recent publication of documents listing 2008 contributors to the National Organization for Marriage. Among those contributors was Mitt Romney.
Both the gay advocacy group the Human Rights Campaign and the Huffington Post posted the documents -- the National Organization for Marriage has claimed it appears someone in the IRS fed the documents to the Human Rights Campaign.
That possibility "is a matter that I take with the utmost seriousness," Hatch wrote, calling the allegation "disturbing."
"Our political history shows the absolute necessity of maintaining the nonpartisan integrity of the IRS," Hatch wrote, calling for an investigation.