The Obama administration has also been forced on the defensive over its handling of last September's terrorist attack in Benghazi, Libya, that killed the US ambassador and three other Americans, and the government's seizure of The Associated Press' telephone records as part of a leaks investigation.
Republicans are hoping to link the issues in an effort to raise questions about Obama's credibility and make it harder for him to press a second-term agenda.
Pfeiffer's appearances were unlike to quiet Republican critics, who have seized on the revelations as proof that Obama used the Internal Revenue Service to go after his political enemies.
"The deputy secretary of the treasury was made aware of just the fact that the investigation was beginning last year," Pfeiffer said. "But no one in the White House was aware."
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A Treasury Department inspector's report said this week that conservative and small-government tea party groups that were critical of Obama received extra scrutiny. IRS agents did not flag similar progressive or liberal groups, according to the watchdog.
The report concluded that a regional IRS office in Ohio improperly singled out tea party and other conservative groups for more than 18 months and took no action on many of their applications for tax-exempt status for long periods of time hindering their fundraising for the 2010 and 2012 elections.
That's not sufficient, said Republican Sen Rob Portman of Ohio. "I think a special counsel is going to wind up being necessary," he added.
That move is not needed, said Democratic Sen Robert Menendez of New Jersey. "I don't see the point," he said of a special counsel to investigate the scandal.