David Coombs, who is representing Pfc. Bradley Manning, argued that Lt. Col. Paul Almanza, a lawyer with the U.S. Justice Department, serving in the Army Reserves, at least appears to be biased, The New York Times reported. Coombs suggested Almanza has already shown prejudice against Manning in some of his rulings.
Almanza halted the pretrial hearing to consider the question.
Manning, who was an Army intelligence analyst, could face a life sentence if he is convicted in a court-martial.
The soldier, arrested in Iraq 19 months ago, pleaded not guilty to more than 20 criminal charges, including aiding the enemy and transmitting national defense information in violation of the Espionage Act.
He is the only person charged with unauthorized release of more than 500,000 classified U.S. military reports and diplomatic cables, as well as a 2007 video of a deadly U.S. helicopter attack in Baghdad, to the WikiLeaks Web site.
Manning's defense team indicated it will try to show that damage to U.S. security because of the leaks was overblown, procedures for securing classified information weren't followed, and signs that Manning suffered from mental health problems that could have made him a security risk were ignored, the Los Angeles Times said.
Coombs, has asked that former Defense Secretary Robert M. Gates and Secretary of State Hillary Clinton testify about the effect of the disclosures, and he wants President Barack Obama to testify about comments he made to a Manning supporter in April.
Legal experts told the Times it was unlikely that senior U.S. officials would be compelled to testify.