Donald Trump is pressing Senate Judiciary Chairman Lindsey Graham to call former President Barack Obama to testify about what the incumbent says was a plot to damage his 2016 candidacy then hobble his presidency.

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

The president fired off that tweet minutes before his top spokesperson, Kayleigh McEnany, appeared on Fox News to call the alleged conspiracy “the biggest political scandal in history.”

She and her boss were referring to what the president has dubbed “Obamagate,” a complex web of intelligence and law enforcement investigations, moves by former Obama administration officials and other initiatives the Trump camp, backed by GOP legislators, that Mr Trump is peddling as he seeks a second term.

The “Obamagate” theory maintains that the 44th president and his top intelligence chiefs sought to entrap incoming Trump officials in legal controversies to kneecap Mr Trump’s presidency from the start. Some aspects of the president’s theory have yet to be corroborated, but he and his top aides and surrogates are pushing the narratives by asking questions about the motives of a number of individuals who were senior members of the Obama administration’s national security, intelligence and foreign policy team.

The White House’s Thursday messaging came a day after Senate Republicans released documents showing a list of former Obama administration officials sought to “unmask” the person on the other end of phone calls with Russia’s then ambassador to Washington. That person turned out to be Michael Flynn, Mr Trump’s 2016 campaign adviser and first national security adviser.

Mr Trump joined Senate Republicans on Wednesday in slamming the former Obama administration officials, including his presumptive 2020 general election foe Joe Biden, of being involved in a “massive” plot to take him down.

“The unmasking is a massive thing,” he said of the former vice president and others in the previous administration seeking to find out who was on the other end of phone calls with Russia’s then-ambassador to the United States. It turns out it was Michael Flynn, his 2016 campaign adviser and first national security adviser.

Mr Trump, speaking to reporters at the White House on Wednesday, accused Mr Biden of lying during a television interview earlier this week during which the former VP said he knew there was an investigation of Mr Flynn but was unaware of specifics.

“He said he knows nothing about anything,” Trump said, before saying Biden was an unmasker and questioning why he could say he knew nothing. “He knows nothing about anything. … And then it gets released today that he’s a big unmasker. How do you know nothing if you’re one of the unmaskers?”

Some Obama allies, however, say the conspiracy theory has many holes. One, they say, is then-President Obama telling Mr Trump in late 2016 when he was president-elect that he had concerns about Mr Flynn taking the national security adviser role.

The president and GOP senators are pressing the former officials to explain why they pushed to learn who the former Russian ambassador, Sergey Kislyak, was talking to between December 2016 and early January 2017. That was the window of time during which Mr Trump was in his transition phase from winning the 2016 election to being sworn in.