Speaking at the National Press Club, Mr. Koskinen pleaded with more money, saying a budget boost would help them staff their overwhelmed customer service lines. He also said it would help reverse staffing cuts in their compliance division, where he said the government will lose $2 billion this year in money it would otherwise have been able to collect if it had better staffing.

Congress has cut or held the agency's funding static for several years now, with lawmakers deeming the agency recalcitrant in solving problems, and unrepentant for having targeted tea party groups for intrusive scrutiny.

The poor customer service is growing worse at the agency. Just a couple of weeks ago Mr. Koskinen had said they were answering 43 percent of phone calls, so dropping below 40 percent suggests things have deteriorated as the April 15 tax deadline nears.