Attorney General Eric Holder would not comment Thursday on whether the Justice Department had launched an investigation into allegations of impropriety in the Pennsylvania Senate race.

Holder said he would not comment on any ongoing or pending investigation in response to aggressive questioning from Rep. Darrell Issa (Calif.), a Republican lawmaker investigating the Pennsylvania race.

"I don't answer hypotheticals," Holder told Issa on Thursday afternoon during testimony before the House Judiciary Committee.

Issa, the ranking member of the House Oversight and Government Reform Committee, has been probing an allegation by Rep. Joe Sestak (D-Pa.) that the White House had approached him about dropping a primary challenge to Sen. Arlen Specter (D-Pa.), in exchange for a top Defense Department job.

Issa had sent a letter to Holder to ask for a special prosecutor in the matter, and said he planned to press the attorney general on the matter after not having received a response.

Holder said he thought the Justice Department had replied and apologized for no response having been received by Issa.

But the attorney general said the situation was out of his hands and wouldn't comment on Issa's line of questioning about whether the type of allegations made by Sestak would constitute a crime.

"I can say that with regard to the appointment of a special prosecutor, that is done on a case-by-case basis," Holder explained. "I have great faith in the people in the public integrity section to handle this matter.

"It is the Department's policy not to comment on pending matters, to say there is an investigation, to say there is not an investigation," Holder later added.

Issa said that Holder's answers put the credibility of the DoJ at risk.

"The very credibility of the Justice Department is compromised when the Attorney General turns a blind-eye to what very well could be a felony perpetrated by someone working inside the Obama Administration," he said in a statement. "Just imagine the kind of outrage that would exist if this same type of bribe was perpetrated by the Bush Administration – would anyone tolerate a no-comment then? I doubt it."

Updated at 3:39 p.m.