Sparks could fly Thursday during Eric Holder's appearance before the Senate Judiciary Committee as he seeks to be confirmed as President-elect Barack Obama's attorney general.
After a series of tame confirmation hearings for uncontroversial nominees, Holder has drawn criticism from Republicans for his role in the 2001 pardon of fugitive financier Marc Rich and the 1999 clemency grants to Puerto Ricans from two militant groups.
Watch FOXNews.com for streaming video of the Senate Judiciary Committee confirmation hearing at 9:30 am. ET Thursday.
GOP lawmakers have indicated they expect to approve Holder. But the hearing promises to be the most contentious so far for Obama's Cabinet choices.
"I think that this (hearing) will probably be … more in depth," one committee aide told FOXNews.com, predicting the questioning could last two days.
The panel's ranking Republican Sen. Arlen Specter, Pa., has already raised flags over Holder's involvement in questionable clemency decisions.
Rep. Dan Burton, R-Ind., attempted to stoke tensions Wednesday when he argued against Holder's confirmation in a letter to the House Judiciary Committee.
House members are not involved in the confirmation of Cabinet secretaries, but Burton cited his time as chairman of the House Government Reform Committee — when Holder was serving as deputy attorney general under Janet Reno in the Clinton administration — for his expertise on the candidate. In the letter, Burton argued that he investigated what he viewed as "politicization" of the Justice Department.
"Eric Holder failed to stand up for the Justice Department, and let down the American people. While your decisions will ultimately be guided by your own review of the matter, in my view, Eric Holder is the wrong choice for attorney general," he wrote.
He specifically targets Holder for his involvement in the Rich pardon, one of a slew of questionable clemency requests Clinton granted at the very end of his term.
Burton said the Rich pardon was "completely indefensible on the merits" and that Holder "stood virtually alone" among Clinton's advisers in supporting it.
Burton also cited recent press reports about Holder's involvement in seeking clemency for 16 members of two Puerto Rican nationalist groups.
The Los Angeles Times reported last week that Holder pushed his subordinates at the Clinton Justice Department to drop their opposition to clemency for 16 members of the violent FALN and Los Macheteros.
Among those scheduled to testify at Holder's hearing Thursday will be Richard Hahn, a retired FBI agent. He said several of the Puerto Ricans freed by Clinton were co-conspirators in planning FALN crimes, which including bombings, robberies and murders.
"These were not activists walking around with signs, these were violent terrorists," Hahn said in an interview with The Associated Press. "And anyone in our government that cared to learn that could have learned by simply contacting the FBI."
From 1975 to 1988, Hahn investigated FALN crimes. He objected to the clemency, he said, because a number of FALN-related crimes remain unsolved, fugitive co-conspirators never were arrested, prisoners showed no remorse and there was no agreement with the prisoners to cooperate with authorities.
Clinton said in 1999 his clemency decision was influenced by South African Archbishop Desmond Tutu and former President Jimmy Carter. At the time, the government said none of those whose sentences commuted was directly responsible for deaths or injuries. That was disputed by Sen. Orrin Hatch, R-Utah, who then led the Judiciary Committee. Republicans complained that under today's sentencing guidelines, the prisoners would have been sentenced to 30 years to life.
In newly disclosed documents, first reported by the Los Angeles Times last week, then-U.S. pardon attorney Roger Adams expressed concerns that the department's recommendation raising the possibility of clemency would become publicly known. "It is particularly important that the proposed report be kept on very close hold," Adams wrote in August 1998 to Kevin Ohlson, who was Holder's chief of staff. "A 'leak' of the proposed report would be disastrous."
Holder was asked by senators in October 1999 whether it was a mistake not to notify bombing victims and their families that President Clinton was considering clemency for the bombers.
"I think we could have done a better job," Holder said.
The Associated Press contributed to this report.