US House Speaker Nancy Pelosi on Sunday demanded that two former attorneys general testify about a "rogue" Justice Department attempt to secretly obtain data from Democrats and news outlets.
"What the [Trump] administration did – the Justice Department, the leadership of the former president – goes even beyond Richard Nixon," Ms Pelosi told CNN.
She was referring to the Watergate scandal that ended the Nixon presidency.
Democrats expressed outrage on Friday over news that the Justice Department under former president Donald Trump secretly watched them investigate possible collusion with Russia.
The department secured the phone records of top political foes and even of family members, including a child, in what Democrats called an unprecedented abuse of power.
The department also seized records of reporters from CNN, The Washington Post and The New York Times without their knowledge.
And on Sunday, the Times reported that the Justice Department in 2018 also subpoenaed the account information of Donald McGahn.
As White House counsel to Mr Trump, Mr McGahn was a key point of contact between him and the department.
The investigation began under attorney general Jeff Sessions and continued under his successor, William Barr. Both recently denied knowledge of any such secret subpoenas.
"This is about undermining the rule of law, and for attorneys general Barr and Sessions to say they didn't know anything about it is beyond belief," Ms Pelosi said on Sunday.
"So, we will have to have them come under oath to testify about that."
Asked if she would subpoena the two men if necessary, Ms Pelosi said she hoped they would "honour the rule of law".
She went on, "the Justice Department has been rogue under President Trump, understand that, in so many respects."
Ms Pelosi said it was essential to determine whether those involved in the operation were still in the Justice Department.
Its targets included intelligence committee chairman Adam Schiff, a Democrat.
"No matter who is president, whatever party, this cannot be the way it goes," she said.
Ms Pelosi also said she would decide on Monday whether to appoint a select committee to investigate the January 6 storming of the US Capitol by Trump supporters.
House-passed legislation to form a bipartisan commission to examine the insurrection was blocked when only a handful of Senate Republicans indicated they would support it.
Ms Pelosi told CNN she promised the Senate that she would give them until Monday to try to find the needed votes for the commission. But most analysts expect that to fail.
If it does fall short, she said she would decide whether to go ahead without Republican support.
"This is about an assault on our democracy, on our Capitol of the United States," she said.
"The American people deserve and must have answers. We will seek the truth. We will find the truth."