A lack of political motivation "is almost beyond belief," said Sen. Mike Crapo, R-Idaho.
George's report blamed ineffective management for allowing agents to inappropriately target conservative groups for more than 18 months during the 2010 and 2012 elections. Shulman was appointed by President George W. Bush and served from March 2008 until last November.
At a separate hearing, Treasury Secretary Jacob Lew said the IRS's actions against conservative groups were "unacceptable and inexcusable."
Lew told the Senate Banking Committee that he has directed the agency's incoming acting director, Daniel Werfel, to hold people accountable and to fix any flaws in IRS management to make sure there is no recurrence of the problems.
Lew said he first learned about the inspector general's investigation in March but that he was unaware of the findings until they became public this month. Lew became Treasury secretary in February, and was White House chief of staff before that.
Shulman said he first learned that something was happening in the spring of 2012.
He said that at that time, he learned that IRS workers were using a list to help decide which groups seeking tax-exempt status should get special attention and knew the term "tea party" was on that list. But he said he didn't know what other words were on that list or the scope and severity of the activity.
He said he took what he thought were the proper steps — making sure the inspector general was looking into the situation. He said he did not tell Treasury officials about the improper activity.
For more than a year, from 2011 through the 2012 election, members of Congress repeatedly asked Shulman about complaints from tea party groups that they were being harassed by the IRS.
Shulman's responses, usually relayed by a deputy, did not acknowledge that agents had ever targeted tea party groups for special scrutiny. At a congressional hearing March 22, 2012, Shulman was adamant in his denials.