Republican attempts to turn the row over the IRS into a scandal engulfing the White House fizzled on Friday when the ousted head of the agency delivered a public apology at a congressional heading into the affair.

Steven Miller, who was fired on Wednesday, portrayed the tougher scrutiny meted out to conservative groups seeking tax-exempt status as the result of overworked employees struggling to cope with a flood of applications, rather than a partisan effort.

In a three-hour hearing at the House ways and means committee, Morris conceded that "foolish" mistakes were made by people trying to be more "efficient".

Despite repeated attempts, Republicans on the committee failed to establish a link between the IRS scandal and either the White House or the 2012 Obama re-election campaign. No new major details about the scandal emerged, offering the White House breathing space going into the weekend after one of the most frenzied weeks since Obama took office in 2009.

At the opening of the hearing, the Republican chairman David Camp accused Barack Obama of hiding the scandal in the run-up to last year's presidential election. "Listening to the nightly news, this appears to be just the latest example of a culture of cover-ups – and political intimidation – in this administration," Camp said. " It seems like the truth is hidden from the American people just long enough to make it through an election."

His comment immediately alienated Democrats on the committee, leaving it divided, with Democrats defending the Obama administration. With the committee split, it lost much of its momentum.

The White House was helped by the testimony of Miller, whom it had sacrificed this week. Miller, who received a hostile reception from the Republicans throughout the hearing and who was repeatedly accused of lying, started with an apology. "I want to apologise on behalf of the Internal Revenue Services for the mistakes that were made," he said.

He portrayed the affair as the result of incompetence by overworked staff at an office in Cincinnati, Ohio, rather than a politically motivated conspiracy. "I do not believe partisanship motivated the people involved in the practices described … I think what happened here was that foolish mistakes were made by people trying to be more efficient in their workload selections," he said.

Miller described what had happened as intolerable but said it had been "a mistake, not an act of partisanship". Further playing down the scandal, he described it as "horrible customer service" and insisted that nothing illegal had happened.

The Republican attacks failed to hit home partly because Miller came across as an old-fashioned public servant caught up in a mess for which he had not personally been responsible.

Camp was among Republicans who accused Miller of lying. "Despite a two-year-long investigation by this committee, the IRS never told the American people or their representatives about this simple truth. That is not being misleading. That is lying," Camp said.

Paul Ryan, Mitt Romney's vice-presidential running mate, was one of the Republicans who pushed Miller the hardest and echoed Camp's point.

Miller denied he had misled them, saying he had been replying at an earlier committee hearing to another point. "I answered the question truthfully," Miller said. "I answered them as they were asked."

Asked by Ryan why then had he resigned, Miller said that, as head of the agency, he took responsibility for the mistake even though he had not been personally involved in this particular episode, an answer that won him the sympathy of some committee members.

Earlier, Miller described the treatment of conservative groups by the IRS as obnoxious but said the IRS had received 70,000 applications for tax-exempt status and only 150 people to deal with them. He suggested appointing more staff.

The Republicans' failure to achieve the political coup they were seeking was down in part to their decision from the outset to make it a partisan issue, alienating Democrats. The Democrats castigated the Republicans for trying to blame Obama and pointing out that some of the key appointees had been made by the Bush administration.

Camp, opening the hearing, said: "Trimming a few branches will not solve the problem when the roots of the tree have gone rotten. And that is exactly what has happened with our entire tax system – it is rotten at the core, and it must be ripped out so we can start fresh." He then added his remark about the alleged culture of cover-up by the Obama administration.

The ranking Democrat on the committee, Sandy Levin, following Camp, rounded on him. "We must seek the truth, not political gain," he said, taking issue with him for suggesting that there were a culture of cover-up.

Republicans turning it into a political issue, as part of the warm-up for the 2014 election would be making a "very, very serious mistake," Levin said, who departed from prepared remarks to deliver the rebuke.