While acknowledging that the IRS bungled tea party applications, and that he himself gave wrong information to Congress, agency Commissioner John Koskinen told lawmakers Wednesday he needs to remain at the IRS in order to clean it up. At a hearing designed to lay the ground for his possible impeachment, Mr. Koskinen implied he was let down by his subordinates, who allowed hundreds of backup tapes to be destroyed, losing tens of thousands of emails from former senior executive Lois G. Lerner. It is the loss of those emails, even as they were under a subpoena issued by Congress, that...