WASHINGTON >> President Donald Trump has embarked on an aggressive new drive to rewrite the narrative of the Russia investigation by making dark and unsubstantiated accusations that former President Barack Obama masterminded a sinister plot to bring him down.

On Twitter, on television, in the Rose Garden and even on an official White House social media page, Trump in recent days has taken aim at his most recent predecessor in a way that no sitting president has in modern times, accusing Obama of undefined and unspecified crimes under the vague but politically charged catchphrase “Obamagate.”

The president went even further today by demanding that Obama be hauled before the Senate “to testify about the biggest political crime and scandal in the history of the USA,” a scenario that itself has no precise precedent in American history. Within hours, Trump’s most faithful Republican ally in the Senate promptly announced that he would indeed investigate, although he would probably not summon Obama.

In flinging incendiary charges at his predecessor, Trump has offered no evidence and has not even specified what “crime” he is accusing the former president of committing. Instead, Trump seemed to be tying the investigation by the special counsel Robert Mueller, which has enraged him for years, back to Obama while hinting ominously at forthcoming revelations that will bolster his claims.

The new focus on the former president comes as Trump appeared eager to change the subject amid the deadliest public health crisis to confront the United States in a half-century. On a day when the death toll in the United States topped 85,000 and the government reported nearly 3 million more people filing for unemployment, Trump spent part of his morning attacking Obama.

In addition to diverting attention from the coronavirus pandemic, Trump’s focus on Obama allows him to try to turn the tables on his accusers by making them out to be the ones who are corrupt while simultaneously putting his Democratic challenger, former Vice President Joe Biden, on the defensive.

“This was all Obama, this was all Biden,” Trump said in an interview on Fox Business Network that aired today. “These people were corrupt, the whole thing was corrupt, and we caught them. We caught them.”

When host Maria Bartiromo asked if he believed that Obama directed U.S. intelligence agencies to spy on him, Trump agreed, without evidence.

“Yes, he probably directed them,” Trump said. “But if he didn’t direct them, he knew everything — and you’ll see that,” he went on, adding that documents would be released soon to bolster his charges.

No evidence has emerged that before the November 2016 election Obama was involved in the FBI investigation into Trump’s advisers and any ties to Russian campaign interference, much less that he directed it, although its existence had been reported in the news media. Obama was told in January 2017 about telephone calls between Trump’s incoming national security adviser, Michael Flynn, and Russia’s ambassador discussing sanctions that the outgoing president had just imposed on Moscow in response to its attempted election sabotage.

Documents released by Trump’s allies this week show that several Obama administration officials, including Biden, requested the identity of the American who was originally unnamed in intelligence reports about contacts with Russia, an American who turned out to be Flynn. Such “unmasking” requests are made thousands of times a year and, according to the documents, these were approved under normal procedures and the recipients were authorized to receive the information.

But Trump’s allies suggest the requests indicated that Obama’s aides must have been involved in trying to “set up” Flynn, who was interviewed by FBI agents after Obama left office and eventually pleaded guilty to lying to them. Attorney General William Barr last week moved to throw the case out, concluding that the FBI had no basis to interview Flynn, a decision that Obama later said undermined the rule of law.

Trump’s attacks have been amplified by Fox News, other conservative media and his reelection apparatus. He has even used his government platform to advance the charges, posting a campaign-style “Obamagate” video on the official White House Facebook page, an overtly partisan message that would have been seen as crossing a line in past administrations.

Trump has often aired grievances against political opponents with sensational but unspecific accusations, leaving advisers to follow and try to fill in the lines. In this case, Trump hopes enough information will be released by his intelligence appointees to muddy the waters and lend a patina of confusion about what Obama may have done, according to people familiar with his thinking.

Trump has long harbored a personal animus toward Obama. Trump spent much of Obama’s presidency championing the racist and false “birther” conspiracy theory that Obama was born in Africa. Obama reciprocated by mocking Trump at the White House Correspondents’ Association dinner in 2011 as the reality show host sat nearby seething.

Trump turned back to Obama in March 2017, two days after Jeff Sessions, then the attorney general, recused himself from the investigation into Russian election interference, a move that infuriated the president and led to the appointment of Mueller.

As Trump stewed at Mar-a-Lago, his Florida club, an aide showed him a story from the conservative website Breitbart quoting radio host Mark Levin accusing Obama of “police state” tactics. The president took to Twitter to assert that Obama “had my ‘wires tapped,’” a claim that Trump’s own Justice Department later debunked.

The president lashed out again last weekend after news reports about Obama’s criticism of Barr’s decision to negate Flynn’s guilty plea.

Trump began using the term “Obamagate” on Sunday but refused to explain what specific crime he was alleging when asked the next day by a Washington Post reporter. “You know what the crime is,” the president said testily. “The crime is very obvious to everybody. All you have to do is read the newspapers, except yours.”

He still has not explained, but this morning, he prodded Sen. Lindsey Graham, R-S.C., the chairman of the Senate Judiciary Committee, into opening an investigation.

“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 wrote on Twitter. “He knew EVERYTHING. Do it @LindseyGrahamSC, just do it. No more Mr. Nice Guy. No more talk!”

Less than two hours later, Graham announced hearings into the Flynn case and other matters, including whether Mueller should have been appointed in the first place. Mueller’s investigation concluded that Russia mounted a major operation to tilt the 2016 election to Trump and that there were multiple contacts with Trump’s associates, but it did not find enough evidence to charge a criminal conspiracy.

“To say we are living in unusual times is an understatement,” Graham said in a statement. “We have the sitting president (Trump) accusing the former president (Obama) of being part of a treasonous conspiracy to undermine his presidency. We have the former president suggesting the current president is destroying the rule of law by dismissing the General Flynn case.”

Still, Graham threw cold water on the idea of calling Obama. “Both presidents,” he added, “are welcome to come before the committee and share their concerns about each other. If nothing else, it would make for great television. However, I have great doubts about whether it would be wise for the country.”