Two Republican lawmakers and a conservative legal group are questioning the Justice Department’s selection of a Democratic donor to lead the agency’s probe into the Internal Revenue Service’s targeting of certain advocacy groups during the 2010 and 2012 election cycles.

House Oversight Committee Chairman Darrell Issa (R-Calif.) and Rep. Jim Jordan (R-Ohio) issued a letter to U.S. Attorney General Eric H. Holder Jr. on Wednesday demanding the department remove DOJ trial attorney Barbara Bosserman from the case, saying her involvement is “highly inappropriate and has compromised the administration’s investigation of the IRS.”

Bosserman donated a combined $6,750 to President Obama’s election campaigns and the Democratic National Committee between 2004 and 2012, according to federal campaign finance records.

The American Center for Law and Justice, which represents 41 groups suing the IRS over its controversial screening methods, also criticized the appointment of Bosserman to lead the probe.

“Appointing an avowed political supporter of President Obama to head up the Justice Department probe is not only disturbing but puts politics right in the middle of what is supposed to be an independent investigation to determine who is responsible for the Obama administration’s unlawful targeting of conservative and tea party groups,” ACLJ chief counsel Jay Sekulow said in a statement Thursday.

The Justice Department said that it cannot take political leanings into account when assigning cases and that making legal political contributions does not prevent its attorneys from fulfilling their duties without bias.

“It is contrary to department policy and a prohibited personnel practice under federal law to consider the political affiliation of career employees or other non-merit factors in making personnel decisions,” said Justice spokeswoman Dena Iverson. “Removing a career employee from an investigation or case due to political affiliation, as chairmen Issa and Jordan have requested, could also violate the equal opportunity policy and the law.”

Efforts to reach Bosserman on Thursday were unsuccessful, and the DOJ did not respond to requests for an interview with her.

An inspector general’s report last year said the IRS applied extra scrutiny to some advocacy groups based on their political beliefs, largely affecting conservative organizations. The audit did not find conclusive evidence of political motivations behind the targeting methods, but Republican lawmakers have suggested that high-ranking administration officials were behind the actions as part of an effort to stifle the president’s critics.

IRS officials apologized for the agency’s actions in May, and former agency commissioner Steven Miller resigned that month amid pressure from the White House. Lois Lerner, who headed the agency’s exempt-organizations division, stepped down in the fall.

The investigation, ordered by Holder, seeks to determine whether criminal violations occurred in connection with the targeting scandal. Several groups have claimed that the IRS leaked confidential information to media and progressive organizations, which is prohibited by law.

The investigative-reporting organization ProPublica revealed in May that the IRS provided confidential tax-exemption applications after it requested the information in 2012. The Huffington Post also reported that year that it received a filing that a whistleblower leaked to the pro-gay rights group Human Rights Campaign.