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Ms. O’Connor spent six months with the IRS and was not at the agency when it realized Ms. Lerner’s emails were missing. She has been at her White House position for one month.

Democrats said government computer crashes are common and Congress should be buttressing agency systems instead of pointing fingers and slashing budgets.

“You can’t have it both ways,” said Rep. Gerald E. Connolly, Virginia Democrat.

Already under the microscope for asking about groups’ religious leanings and donors, the IRS is losing more credibility with its lost emails.

Ms. Lerner has cited her Fifth Amendment rights and refused to cooperate with the congressional probe, leading Republicans to hold her in contempt.

Democrats shared disgust with some of the IRS practices but reserved much of their criticism for their Republican counterparts.

“Today’s hearing is not about policy or substance,” said Rep. Elijah E. Cummings of Maryland, the panel’s ranking Democrat. “It’s about politics and press.”

Ben Wolfgang contributed to this report.