US Attorney General William Barr said he did not expect an investigation into the origins of the FBI's Russia investigation to lead to criminal probes of either President Donald Trump's Democratic opponent Joe Biden or former President Barack Obama.
Mr Trump has stated without evidence that he believes Mr Obama committed unspecified crimes as president, repeatedly tweeting, "OBAMAGATE!"
The claims have become a rallying cry among his supporters, while Democrats view it as a desperate attempt to shift the focus from the president's handling of the coronavirus outbreak and the nation's soaring unemployment.
Mr Barr insisted the Justice Department would not be swayed by political pressure to investigate the president's opponents and the "criminal justice system will not be used for partisan political ends".
His comments come as Democrats and some former law enforcement officials have accused the attorney general of politicising decisions and doing Mr Trump's bidding at the Justice Department.
The criticism was stepped up two weeks ago when the department moved to dismiss charges against Mr Trump's former national security adviser Michael Flynn, who had pleaded guilty to lying to the FBI.
"We live in a very divided country right now and I think that it is critical that we have an election where the American people are allowed to make a decision, a choice, between President Trump and vice president Biden based on a robust debate of policy issues," Mr Barr said.
"And we cannot allow this process to be hijacked by efforts to drum up criminal investigations of either candidate."