(CNN) -- Just when it seems impossible for things to get uglier between the two heads of a key House investigative committee, they have.
Hours after Democrat Elijah Cummings accused House Oversight Chairman Darrell Issa of engaging in "McCarthyism," Issa shot back with a letter to Cummings accusing him of obstructing the committee's IRS investigation, and even contributing to the agency's targeting of conservative grass roots groups.
"New IRS documents identified by the committee raise disturbing concerns about your possibly motivations for opposing this investigation and unwillingness to lend your support to efforts to obtain the testimony of former IRS exempt Organizations Director Lois G Lerner," Issa wrote to Cummings.
Issa and fellow Republicans on the committee plan to hold a vote Thursday holding Lerner in contempt of Congress.
Democrats say Republicans are trampling on Lerner's constitutional rights to remain silent, and released a Congressional Research Service report saying the only time in modern history Congress held witnesses in contempt was in the 1950s during the anti-communist McCarthy era, one of the darkest times in congressional history.
At the heart of Issa's new allegations about Cummings are letters sent to a conservative group called True the Vote asking for information about the group.
The head of True the Vote said Cummings tactics were the same as those used by the IRS, which she called "unacceptable."
In addition, Issa is releasing what he says is new evidence Cummings contacted the IRS that Issa says may have helped spur inappropriate IRS questions to True the Vote.
"Several of your requests are virtually identical to the information requests sent by the IRS to True the Vote in February 2012," wrote Issa.
"This timeline and pattern of inquiries raises concerns that the IRS improperly shared protected taxpayer information with your staff," Issa wrote Cummings.
Issa is also accusing Cummings of withholding information from committee Republicans about his communications with the IRS and True the Vote.
"The American people deserve to know the full truth. They deserve to know why the ranking member and minority staff of the House Committee on Oversight and Government Reform surreptitiously contacted the IRS about an individual organization without informing the majority staff and even failed to disclose the contact after it became an issue during a subcommittee proceeding," said Issa.
Cummings points out that he only requested public information from the IRS, and that he posted his inquiry to True to the Vote on his website -- so he wasn't hiding anything.
"I have made no secret of my concern about True the Vote's political activities," wrote Cummings.
Cummings argues that in the case of True the Vote, there was legitimate concern about IRS granting tax-exempt status, since the group had given $5,000 to a state Republican organization, which tax-exempt groups are banned from doing.
"This political organization has raised millions of dollars for Republican candidates and describes itself as "the largest caucus of Republican state leaders and the only national organization whose mission is to elect down ballot, state-level Republican office-holders," said Cummings, in a response letter to Issa.
CNN's Craig Broffman contributed to this report