Washington (CNN) -- In the latest salvo of a pitched political battle over Internal Revenue Service targeting, the top Democrat on the House Oversight Committee said Tuesday that interviews with 15 tax agency employees found no evidence of bias or White House manipulation alleged by Republicans.
"Despite an extremely aggressive investigation involving thousands of documents and more than a dozen interviews of IRS employees, the overwhelming evidence before the committee reveals no political motivation or White House involvement in this process," said the memo made public by Rep. Elijah Cummings of Maryland, the panel's ranking Democrat.
The committee chaired by Republican Rep. Darrell Issa of California will hold another hearing on the IRS targeting on Thursday, continuing a probe that has bitterly divided its two party leaders.
Cummings accuses Issa of conducting a partisan witch hunt and making false accusations about the scandal first uncovered in May by an inspector general's report that detailed how the IRS targeted conservative groups seeking tax-exempt status for extra scrutiny.
Issa and other Republicans have claimed the targeting amounted to a campaign by President Barack Obama's White House to go after political foes, with some comparing it to the Watergate scandal that forced President Richard Nixon from office.
Democrats have pushed back, led by Cummings.
Last week, he revealed new documents indicating the IRS subjected both liberal and conservative organizations to additional tax scrutiny in recent years.
The memo Cummings made public Tuesday was written by the committee's Democratic staff. It contained excerpts of interviews with 15 IRS employees from the committee's investigation of the targeting cited by the inspector general's report.
In the excerpts, the employees who identified themselves as Republicans, Democrats and independents said there was no political motivation or outside influence involved in the agency's handling of tax-exempt requests from groups with possible or likely political affiliations.
Under tax law and IRS regulations, groups that primarily engage in political activity are ineligible for tax-exempt status.
According to the Cummings memo, an IRS tax law specialist based in Washington who described herself as Republican said "no, not at all" when asked if there was any evidence that the agency targeted Obama's political enemies.
"That's kind of laughable that people think that," the memo quoted the woman as saying.
She described the situation as a lack of guidance regarding how to deal with politically affiliated groups seeking tax-exempt status at the IRS unit based in Cincinnati that handles such applications.
Thursday's hearing promises to be a fiery showdown between Cummings and Issa over the initial inspector general's report and the subsequent findings and accusations by both sides.
According to the House website, the inspector general for tax administration, who wrote the report on IRS targeting, will be a witness along with past and present IRS employees in the tax-exempt division.
Cummings demanded last week the inspector general, J. Russell George, be called to appear again before the committee to answer questions about why his original report failed to note that liberal groups also were targeted for extra scrutiny.
The IRS probe "has been characterized by one-sided and partial information leading to unsubstantiated accusations with no basis in fact," Cummings said.
George's report in May indicated that lax oversight at the IRS allowed for the singling out of some conservative groups starting in 2010 and continuing until last year.
In particular, the report said IRS workers in the tax-exempt unit used "Be on the Lookout" or BOLO lists of words such as tea party to assess what applicants came under extra scrutiny.
George subsequently testified that he could not specifically identify whether any liberal groups were similarly targeted. However, Cummings said the information made public last week showed that George was aware that liberal groups also were targeted.
In addition, Cummings revealed a May 2013 e-mail from George's deputy that noted how 5,500 internal IRS e-mails were reviewed as part of an effort to discover any directing of staff to target "Tea Party and other political organizations," as well as any subsequent coverup.
"There was a 'Be On the Lookout' (BOLO) list specifically naming these groups," the deputy inspector general's e-mail notes. "However, the e-mails indicated the organizations needed to be pulled because the IRS employees were not sure how to process them, not because they wanted to stall or hinder the application."
The deputy's e-mail goes on to stress that "there was no indication that pulling these selected applications was politically motivated."
In response to the documents Cummings made public last week, Issa's office said conservative groups underwent tighter scrutiny than liberal groups.
"These documents, once again, refute misleading attempts to equate routine scrutiny of other groups involved in advocacy to the systematic scrutiny of Tea Party groups by IRS officials," spokesman Ali Ahmed told CNN.
"As has been documented, while 100% of Tea Party applications were systematically stopped and scrutinized for a 27-month period, at the same time dozens of progressive applications were approved by the IRS," Ahmed said.
CNN's Alan Silverleib contributed to this report.