Portman's letter comes as the White House seeks to do damage control after the IRS admitted that employees in the agency's Cincinnati field office had been using a higher level of scrutiny of Tea Party organizations.
A number of lawmakers have vowed to hold congressional investigations into the matter. Steve Miller, the acting commissioner of the IRS, resigned on Wednesday.
On Thursday, the White House announced that Office of Management and Budget Controller Danny Werfel would succeed Miller as acting commissioner.
Read the letter below: