President Trump on Thursday escalated his attacks against former President Obama over what Trump has viewed as wrongdoing against his campaign by officials in the previous administration, saying that Republicans should call on the former president to testify.

“If I were a Senator or Congressman, 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. “He knew EVERYTHING.”

The president specifically singled out Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.), telling him to “just do it” and to stop playing “Mr. Nice Guy.”

Graham’s panel is investigating the origins of the FBI inquiry into Russian interference in the 2016 election and links between the Trump campaign and Russia. The GOP chairman has forecast plans to call former senior Obama administration officials to testify, but signaled Thursday he was concerned about the precedent it would set calling Obama himself to testify.

“As previously stated, there will be oversight of all things related to Crossfire Hurricane," Graham said after Trump's tweet, referring to the name given to the FBI's Russia probe. 

 “I am greatly concerned about the precedent that would be set by calling a former president for oversight," he continued.

"No president is above the law. However, the presidency has executive privilege claims against other branches of government," Graham added. 

Trump over the last week has amplified unsubstantiated claims that Obama administration officials working on the Russia investigation improperly targeted his campaign for political reasons.

The president suggested on Twitter over the weekend that Obama was directly involved in the effort and guilty of criminal wrongdoing, though on Monday he declined to name the specific crime he was accusing the former president of committing.

“You know what the crime is. The crime is very obvious to everybody,” Trump told reporters when asked about the tweets.

Critics, meanwhile, have accused the president of attempting to create a controversy in order to distract from his handling of the coronavirus pandemic.

Trump's conservative allies have seized on new revelations related to the criminal case against former White House national security adviser Michael Flynn, claiming they prove that Flynn was wronged by government investigators. 

On Wednesday, Republicans released a list of names declassified by acting Director of National Intelligence Richard Grenell showing that former Vice President Joe Biden and other top Obama officials requested the “unmasking” of Flynn’s name in intelligence reports. Biden is Trump’s presumptive Democratic foe in the 2020 presidential election.

Flynn pleaded guilty in December 2017 to lying about his contacts with the Russian ambassador regarding sanctions but has since sought to withdraw that plea, arguing he was the victim of prosecutorial misconduct. The Justice Department last week moved to withdraw its case against Flynn, a decision that has triggered concerns about politicization of the Justice Department.