President Trump on Thursday urged Senate Judiciary Chairman Lindsey O. Graham (R-S.C.) to call former president Barack Obama to testify about what Trump alleges without evidence was a conspiracy involving his predecessor — a prospect Graham promptly rejected.

In recent days, Trump has sought to amplify an “Obamagate” conspiracy theory that has taken root on the right that falsely suggests Obama and former vice president Joe Biden oversaw an effort to spy on Trump’s campaign to delegitimize 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,” Trump said in a tweet in which he urged Graham to “just do it.”

“No more Mr. Nice Guy. No more talk!” the president wrote.

Trump has called the alleged scandal “worse than Watergate” and has kept up a drumbeat of accusations that Obama committed a crime, though he declined to offer specifics when asked to explain the extraordinary charges during a Rose Garden news conference Monday.

Asked about Trump’s latest tweet on the issue, Graham told Politico that he has no intention of summoning Obama to Capitol Hill.

“I don’t think now’s the time for me to do that. I don’t know if that’s even possible,” Graham said, adding: “I understand President Trump’s frustration, but be careful what you wish for.”

In a subsequent tweet, Graham wrote that both Trump and Obama are “welcome to come before the committee and share their concerns about each other.”

“If nothing else it would make for great TV. However, I have great doubts about whether it would be wise for the country,” the senator added.

The Justice Department said in a published opinion last year that former presidents cannot be compelled to testify before Congress.

Trump has ramped up his accusations in the wake of the Justice Department’s decision to drop its prosecution of former national security adviser Michael Flynn.

Former Obama aides have been dismissive of Trump’s efforts.

“No one knows what ‘Obamagate’ is, and that includes President Trump. It’s a hashtag in search of a scandal,” said Ned Price, a former CIA analyst who served as a National Security Council spokesman under Obama. “The only people it will galvanize are those already 1,000 percent behind” Trump.

David Nakamura contributed to this report.