WASHINGTON » A senior White House aide said Sunday that President Barack Obama learned only from news reports that an Internal Revenue Service office had singled out dozens of conservative groups for questionable scrutiny, while Republicans vowed to investigate any White House involvement in the growing scandal.

White House senior adviser Dan Pfeiffer made the rounds of the Sunday talk shows to vigorously defend Obama in the IRS case, the attacks that killed four U.S. government employees in Benghazi, Libya, and a federal prosecutor’s obtaining phone records of dozens of reporters and editors at the Associated Press.

Republican critics argue that the cases reflect a president out of touch and a White House out of control during the 2012 election season, charges that have energized the GOP and have put Obama on the defensive barely five months into his second term.

The president and his aides have condemned the IRS misbehavior, denounced the Republican inquiries into the Benghazi attack as blatantly partisan, and defended the investigation of the AP as necessary to determine who leaked classified information.

Pfeiffer insisted that Obama had no advance knowledge that federal investigators had begun investigating the IRS misdeeds until he learned about it from media reports on May 10.

It is a “cardinal rule … for all White Houses is you do not interfere in an independent investigation and you do not do anything to give off the appearance of interference in an independent investigation,” Pfeiffer said on CNN.

“The activity was outrageous and inexcusable, and it was stopped and it needs to be fixed to ensure it never happens again,” Pfeiffer said. He said anyone “who did anything wrong will be held accountable.”

Sen. Rand Paul, R-Ky., asserted on CNN that a written policy had directed the IRS to target conservative groups, and vowed to find out “who wrote the policy and who approved the policy.” But he acknowledged that he had no direct knowledge that a written policy existed.

Senate Minority Leader Sen. Mitch McConnell, R-Ky., said the IRS case reflected a “culture of intimidation” in the administration. But he acknowledged on NBC’s “Meet the Press” that no evidence yet indicates that the White House ordered the behavior or tried to cover it up.

Rep. Dave Camp, R-Mich., who heads the House committee looking into the IRS misconduct, also said he has no evidence to suggest Obama had advance knowledge of the IRS actions.

Sen. Rob Portman, R-Ohio, said on ABC’s “This Week” that he believed it would be necessary to appoint a special counsel, independent of administration control, to unearth the facts.

A report released last week by a Treasury Department inspector general said an IRS unit based in Cincinnati had used “inappropriate” and “politically sensitive” criteria to scrutinize conservative groups that had applied for tax-exempt status, delaying action on some applications for more than 18 months.