Later in the election cycle, former aides to President Obama formed their own groups, Priorities USA and Priorities USA Action, which used the same organizational principle to boost the Obama campaign.
McConnell highlighted reports of harassment by the IRS of conservative groups.
"Earlier this year, dozens of Tea Party-affiliated groups across the country learned what it was like to draw the attention of the speech police when they received a lengthy questionnaire from the IRS demanding attendance lists, meeting transcripts and donor information," McConnell said in a speech at the American Enterprise institute last year.
"One of the group's leaders described the situation this way: '[Groups like ours] either drown … in unnecessary paper work … or you survive, and give them everything they want, only to be hung.'"
The rare apology in some ways vindicates conservative groups that have been hounded by allegations that their activities were not completely legal.
It also suggests that the IRS might be attempting to get ahead of legal action or congressional scrutiny, Gross said.
"The IRS is not accustomed to apologizing for anything," Gross said. "Maybe they were trying to avert embarrassing congressional hearings or just get out in front of any criticism that might come their way. But in Washington, these things tend to fuel things rather than abate them."