The chairman of a powerful committee in Congress is demanding access 'without delay' to a key witness in the Lois Lerner email saga that has engulfed the IRS, but the Treasury Department insists she can't testify until after the midterm congressional elections.

House Ways and Means Chairman Dave Camp, a Michigan Republican, demanded late Wednesday in a letter to Treasury Secretary Jacob Lew that he must make Treasury counsel Hannah Stott-Bumsted available before that Nov. 4 political milestone date.

Stott-Bumsted was the first Treasury aide to learn that the IRS was unable to locate two years' worth of disgraced official Lois Lerner's emails because of a hard drive crash.

Her former private law practice colleague Catherine Duval told her in April, according to reports of closed-door testimony Duval gave the House Oversight and Government Reform Committee.

That disclosure, Duval reportedly told the committee, led to the White House finding out about the email losses two months before Congress was notified.

SCROLL DOWN FOR VIDEO AND TO READ THE LETTER

Former IRS Director of Exempt Organizations Lois Lerner has refused to answer lawmakers' questions about the tea party targeting scandal; she was held in Contempt of Congress this year

House Ways and Means Committee chairman Rep. Dave Camp is trying to force the Treasury Department's hand so he can hear testimony from a Treasury lawyer who was the first official outside the IRS to learn that Lerner'shard drive crash had wiped out two years' worth of emails

Lerner, who took a retirement package late last year and refused to answer questions in two separate congressional hearings, remains at the center of a scandal involving alleged political targeting of conservative nonprofit groups when they sought tax-exempt status.

Camp has sought Stott-Bumsted's testimony since mid-September, he wrote, but Teasury has stonewalled him and ignored his requests.

'Treasury officials ignored this committee's phone and email inquiries about Ms. Stott-Bumsted's availability,' Camp wrote to Lew.

'On the morning of October 14, a month from the initial request, the same Treasury officials told staff that Ms. Stott-Bumsted was not available until sometime in November.'

Camp added that Treasury insisted interviewing a Treasury Department lawyer would introduce 'Constitutional and practical concerns' – even though Ways and Means has already questioned more than a dozen of them.

'Why the sudden change in protocol?' he asked.

His demands include a list of everyone at Treasury who learned about the email problems before Congress was notified. He also wants to know who told the White House, which White House officials were informed, and when.

'Your office is now refusing to make available until after the election the very person that could unlock that mystery. This is completely unacceptable, especially for an administration that once pledged to be the most open and transparent ever.'

Hot seat: Treasury Secretary Jacob Lew's agency wants to hold off providing Hannah Stott-Bumsted for testimony until after the midterm elections, when more bad news could hurt Democrats' already waning hope of gaining seats in Congress

The scandal broke wide open in May 2013 when Lerner, then director of the IRS Exempt Organizations division, answered a planted question during a tax lawyers' conference about how her agency decided which nonprofit groups to scrutinize carefully.

Lerner knew what Congress did not: that an internal Inspector General report on the matter would be issued soon, thrusting it into the public spotlight.

Republicans howled at the admission that organizations were selected for intrusive questions and long delays based on words like 'patriots' and 'tea party' in their names.

Days later, President Barack Obama accepted the resignation of the acting IRS commissioner and promised in a statement from the White House that his administration would 'work with Congress as it performs its oversight role' and work 'hand in hand with Congress to get this thing fixed.'

Many of the Lerner emails sought by Congress have been recovered by searching the archives of other IRS employees who sent or received them. But countless more, including any between Lerner and officials at the White House or other agencies, are likely gone for good.

IRS Commissioner John Koskinen has testified repeatedly on Capitol Hill that his employees did everything possible to recover data from Lerner's hard drive, including sending it to forensic experts at the FBI.

Ultimately, he has said, no data was recoverable – and the hard drive was destroyed by a recycling contractor.