A day earlier, Cummings released a memo that cited excerpts from committee interviews with 15 IRS workers that found no evidence of political bias or White House manipulation alleged by Republicans.
"Despite an extremely aggressive investigation involving thousands of documents and more than a dozen interviews of IRS employees, the overwhelming evidence before the committee reveals no political motivation or White House involvement in this process," said the memo written by the committee's Democratic staff.
Issa conceded in an opinion piece published Wednesday on the USA Today website that the two-month investigation has yet to find hard evidence of involvement in the targeting by anyone outside the IRS. He argued that the lack of proof does not mean the investigation should end.
"We candidly still don't have clear answers to many important questions and have yet to begin interviewing senior IRS officials," Issa wrote, saying "judgment should be withheld until all relevant witnesses are interviewed and all documents reviewed."
In an editorial Thursday, USA Today questioned the value of the ongoing investigation.
"No political operatives from the Obama campaign or the White House have been linked to any of the IRS' activities," the editorial said. "What's more, it has become increasingly clear that confusion on the part of IRS employees, rather than a starkly political motive, was the primary cause of the delays."
At times during Thursday's hearing, Issa took a conciliatory stance, saying that any evidence of targeting -- whether of liberal or conservative groups -- should be investigated.
"If the facts are that people got abused for myriad reasons, we ought to know it," Issa said.
Both Issa and Cummings asked George to provide more information on groups targeted by the IRS, and the inspector general pledged to do so.
Cummings and other Democrats took aim Thursday at past statements by Issa and other Republicans, calling them blatantly political accusations with no basis in fact that went against the spirit of Issa's call at the start of Thursday's hearing to "reject, categorically, assumptions for which there is not evidence."
Rep. Gerald Connolly, a Virginia Democrat, called for Issa to retract his earlier comments that the targeting involved only conservative groups and was an effort to target Obama's political enemies.
Issa responded that tea party groups that came under extra scrutiny could be considered political enemies of the president and that he had yet to see hard evidence that liberal groups were targeted.
Republicans made a point Thursday of singling out White House spokesman Jay Carney for criticism, noting he previously said that the targeting involved rogue IRS employees in Cincinnati. Hofacre described herself as offended by the characterization of the issue as misdeeds at her level.
She and Hull said the delays in rendering final decisions on applications by conservative groups was because of a lack of guidance from superiors in Washington.
Under tax law and IRS regulations, groups that primarily engage in political activity are ineligible for tax-exempt status. The targeting occurred after the 2010 ruling by the Supreme Court that opened the political process to more private contributions from business and labor, leading to increased applications for tax-exempt status from groups involved in political activity.
In the memo Cummings made public Tuesday, IRS employees who identified themselves as Republicans, Democrats and independents said there was no political motivation or outside influence involved in the agency's handling of tax-exempt requests from groups with possible or likely political affiliations.
According to the memo, an IRS tax law specialist based in Washington who described herself as Republican said "no, not at all" when asked whether there was any evidence that the agency targeted Obama's political enemies.
"That's kind of laughable that people think that," the memo quoted the woman as saying.
Cummings and other Democrats complained Thursday that the other IRS employees who had been interviewed were not called as witnesses to the latest hearing.