The request, made by PETA's general counsel, Jeffrey Kerr, comes as lawmakers begin to investigate the IRS's heightened scrutiny of Tea Party groups applying for tax-exempt status. The letter is addressed to Eric Thorson, inspector general for the Treasury Department, and J. Russell George, the inspector general for tax administration at the Treasury Department.

"I am writing to request that you please expand your current inquiry into reported Internal Revenue Service misconduct to include a detailed analysis of the three baseless audits that targeted PETA's tax-exempt status in 1990 to 1992 under the George H.W. Bush administration, in 2003 to 2005 under the George W. Bush administration, and again in 2009," Kerr writes in the letter released Thursday. "I can assure you that targeted misconduct by the IRS is neither new nor limited to conservative causes."

The wrongful audit, Kerr writes, happened between 2003 and 2005.

"PETA's harassment by the IRS includes the 20-month audit in 2003 to 2005 and another in 2009, both of which resulted from what the IRS agents admitted—and we have verified from Freedom of Information Act materials—were politically motivated attacks and pressure by members of Congress who were doing the bidding of the meat, dairy, experimentation, tobacco, and other industries whose animal-abusing practices PETA opposes," Kerr continues in the letter.

"PETA came through all three audits with a clean bill of health but endured an unconscionable diversion of charity resources to fend off these attacks on its tax-exempt status, which were reminiscent of the Nixon years and tactics more commonly attributed to totalitarian regimes."

Earlier on Thursday, the White House announced that Danny Werfel would succeed Steven Miller as acting commissioner of the IRS. After revelations of the IRS's targeting of Tea Party groups surfaced, a number of lawmakers called for Miller's resignation.