Attorney General Bill Barr said the original investigation into possible collusion between the Trump campaign and Russia in the 2016 election was a "grave injustice" to the president and based on "utterly baseless" suspicions.
But he said he aimed to stop a cycle of using the US justice system for what he called political investigations, including of Democrat Obama and Biden, who is Trump's rival in the upcoming presidential vote.
Whatever the involvement of Obama or Biden in the Russia collusion probe, Barr said, "based on the information I have today, I don't expect (it) will lead to a criminal investigation of either man."
"It is critical that we have an election where the American people are allowed to make a decision between President Trump and vice president Biden based on a robust debate of policy issues," he told reporters.
"We cannot allow his process to be hijacked by efforts to drum up criminal investigations of either candidate."
The comments came after Trump and members of the Republican party and White House staff began pushing the idea that there was criminal activity at the highest levels behind the 2016-18 counterintelligence investigation into whether Trump's election campaign colluded with Russia.
Trump has repeatedly called for an investigation into his predecessor, calling the alleged scandal "Obamagate."
Last week Trump urged Senator Lindsey Graham, who chairs the powerful Senate judiciary committee, to call Obama to testify.
"The first person I would call to testify about the biggest political crime and scandal in the history of the USA, by FAR, is former President Obama," Trump tweeted.
Barr, who himself has been accused of bending the US Justice system to defend Trump, nevertheless rejected that approach.
"Over the past few decades there have been increasing attempts to use the criminal justice system as a political weapon. The legal tactic has been to gin up allegations of criminality by one's political opponents based on the flimsiest of legal theories," Barr said.
"This is not good for our political life, and it's not good for the criminal justice system."
"As long as I'm attorney general the criminal justice system will not be used for partisan political ends. And this is especially true for the upcoming elections in November."