A number of prominent lawmakers have begun calling for a special counsel to investigate the IRS targeting of conservative groups seeking tax exemption for higher scrutiny.

On Sunday, Sen. Rob PortmanRobert (Rob) Jones PortmanTax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today Doctors bristle at push for opioid prescription limits MORE (R-Ohio) said he believed a special counsel would be "necessary."

The IRS publicly apologized for its actions, which brought criticism from both parties and the White House. But GOP lawmakers are now pressing to know when senior officials at the White House and Treasury first learned about the political targeting.

Davis also expressed disappointment with IRS acting Commissioner Steven Miller’s testimony before Congress on Friday, saying that he appeared “out of his league.”

"I was convinced that he was well meaning, but I also felt that he was a little bit out of his league," Davis said. "He didn't give one a feeling that he was firmly etched as the boss of that unit."

Miller, who submitted his resignation last week, appeared before the House Ways and Means Committee on Friday at an often-contentious hearing to answer lawmaker questions about the IRS scandal.