(CNN) — WASHINGTON (CNN) -- In this corner, the Republican chairman of the House Oversight Committee who alleges without proof that the Internal Revenue Service targeting scandal has links to the White House.
And in this corner, the panel's top Democrat who accuses the GOP chairman of leading a witch hunt for political motives that don't exist.
The two congressional heavyweights will square off Thursday at another committee hearing on the IRS targeting of groups seeking tax-exempt status for extra scrutiny -- the third by the oversight panel and eighth overall by Congress on an issue that has become a symbol of partisan friction.
Rep. Darrell Issa, the California Republican who heads the panel, sent the IRS a letter on Wednesday that made new demands for all communications on the matter between the tax agency, the Treasury Department and Obama's office since February 2010.
The letter signed by Issa and other House Republicans quoted excerpts of an interview with an IRS lawyer who said the office of the IRS chief counsel -- who was appointed by President Barack Obama in 2009 -- played a role in scrutiny that delayed processing tax-exempt applications from some conservative groups.
"Its involvement and demands for information about political activity during the 2010 election cycle appears to have caused systematic delays in the processing of tea party applications," the GOP letter said.
In response, Democratic Rep. Elijah Cummings of Maryland issued his own letter that accused Issa of dishonest intentions.
"Rather than describing the whole truth, your letter appears yet again to create a skewed account based on partial, incomplete, and cherry-picked information while disregarding key evidence that contradicts your political narrative," Cummings wrote.
A day earlier, Cummings released a memo that cited excerpts from committee interviews with 15 IRS workers that found no evidence of political 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 written by the committee's Democratic staff.
Cummings and Issa will square off again at Thursday's hearing, where IRS employees who handled the tax-exempt applications will testify along with the inspector general whose report in May first revealed the targeting of conservative groups for extra scrutiny.
Issa and other Republicans have claimed the targeting amounted to a campaign by the White House to go after political foes, with some comparing it to the Watergate scandal that forced President Richard Nixon from office.
Cummings says Issa is playing politics with the committee's investigation of the targeting,which conservatives repeatedly cite to portray the Obama administration as a government gone wild in overstepping its authority.
Last week, Cummings demanded that J. Russell George, the Treasury inspector general for tax administration, 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.
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. However, it also said no evidence existed that the targeting was politically motivated.
George subsequently testified that he could not specifically identify whether any liberal groups were similarly targeted. However, Cummings said information made public last week showed that George, who was appointed in 2004 by GOP President George W. Bush, was aware that liberal groups also were targeted.
On Wednesday, the acting IRS head named by Obama to clean up the mess told Cummings that George had objected to plans to turn over a document that included information on liberal groups that also came under extra scrutiny.
Daniel Werfel, who took over leadership of the 85,000-employlee IRS on May 22, said a subcommittee hearing that his agency's experts and George disagreed on whether details in the document sought by Cummings could be made public.
Werfel told Cummings he would produce the requested document if George dropped his objection.
At the White House, spokesman Jay Carney told reporters that Obama's interest was in making sure that the IRS operated properly, "and when there are indications and revelations that that is not happening, he has always insisted that action be taken to correct it."
"Others try to make this about politics. Others issue conclusions before facts are known," Carney said. "And that's not helpful to the process."
In the memo Cummings made public Tuesday, IRS 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 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.
Last week, 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' 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.