Republican politicians have called for a broad investigation of the US tax agency's acknowledgement that its agents had singled out conservative political groups for more scrutiny, and demanded that President Barack Obama make clear the action was unacceptable.
In a practice that drew complaints during the 2012 general election campaign, non-profit advocacy groups with the words "Tea Party" or "patriot" in their names were flagged for closer review by the Internal Revenue Service (IRS) when they applied to the agency for tax-exempt status.
"This is something we cannot let stand. It needs to have a full investigation," Mike Rogers, chairman of the House of Representatives Intelligence Committee, said on Fox News Sunday.
"I do not care if you are a conservative, a liberal a Democrat or a Republican, this should send a chill up your spine."
A Republican senator also criticised Obama for not personally condemning the IRS's targeting of conservative political groups during the 2012 election.
Susan Collins, Republican senator from Maine, said Obama "needs to make crystal clear that this is totally unacceptable".
"This is truly outrageous and it contributes to the profound mistrust that the American people have in government," she said on CNN's State of the Union.
Collins said she also doubted the IRS's claim that the groups were targeted as part of an "inappropriate" organising technique by a few bureaucrats in the agency's tax-exempt section, rather than for political reasons.
"I just don't buy that this was a couple of rogue IRS employees. After all, groups with "progressive" in their names were not targeted similarly," Collins said.
"If it had been just a small group of employees, then you would think that the high-level IRS supervisors would have rushed to make this public, fire the employees involved and apologised to the American people and informed Congress," she said.
The tax agency blames low-level employees and says no senior officials were aware of the extra scrutiny to see if conservative groups were violating their tax-exempt status.
But a draft of a watchdog's report obtained on Saturday by the Associated Press news agency says senior officials knew agents were targeting Tea Party groups as early as 2011.