A declassified document with names of President Barack Obama administration officials who made requests for unmasking of…
A declassified document with names of President Barack Obama administration officials who made requests for unmasking of Michael Flynn's name is photographed May 13, 2020.

U.S. President Donald Trump's long-standing accusation of criminality against his Democratic predecessor, Barack Obama, has taken on a new name: Obamagate.

In this new iteration being hotly promoted by Trump and his right-wing allies, Obama officials in the waning days of his administration conspired to entrap Trump's national security adviser, Michael Flynn, as part of a larger plot to bring down the incoming president.

The conspiracy theory took on new life after the Justice Department, in a dramatic about-face last week, moved to drop its criminal case against Flynn, saying the FBI was not justified in investigating him over his 2016 conversations with Russia's former ambassador to Washington.

FILE - Former Trump national security adviser Michael Flynn leaves the federal courthouse in Washington, following a status hearing, July 10, 2018.

As part of its reassessment of the Flynn case, the Justice Department released a deluge of records that shed light on previously unknown Obama administration deliberations over Flynn. That led Trump to claim that Flynn had been illegally targeted and that the decision to investigate him went all the way up to the previous president.

"The biggest political crime in American history, by far!" Trump said in one of a blizzard of tweets and retweets referencing Obamagate on Sunday.

In short order, the hashtag "Obamagate" went viral, giving an old conspiracy theory a new twist.

Asked on Monday to describe Obama's alleged crime, Trump would only offer that "some terrible things happened, and it should never be allowed to happen in our country." He predicted further disclosures in the coming weeks.

Obama is a favorite and frequent target of Trump's attacks. Trump turned up the heat on his predecessor after Obama, in leaked comments to former officials in his administration, blasted the Justice Department's decision to let Flynn off the hook, saying the "rule of law is at risk."

By advancing new allegations against the former president, Trump is also apparently seeking to implicate his likely opponent this fall, Obama's vice president, Joe Biden.

Here is a primer on the controversy.

Obamagate origins

At its core, Obamagate is an old allegation given a new name.

As early as March 2017, Trump alleged that Obama had illegally wiretapped the billionaire businessman at Trump Tower during the 2016 presidential election campaign, comparing the alleged surveillance to the Nixon-era Watergate scandal. The following year, Trump claimed that the FBI had planted an informant inside his campaign, dubbing the alleged conspiracy "Spygate."

FILE - Russia's ambassador to the U.S. Sergey Kislyak, speaks with reporters in Washington, Jan. 13, 2017.

The main allegation in Obamagate is that the former president directed the Flynn investigation, even though the FBI had no legitimate reason to probe the retired three-star general. Flynn, a one-time Democrat, served as the Obama administration's top military intelligence official before being forced out of the job and later aligning with Trump.

Flynn was investigated twice in 2016 and 2017, first as part of the FBI's probe into ties between the Trump campaign and Russia, and later over a series of conversations he had with Russia's then-ambassador to Washington, Sergey Kislyak, in which he counseled the Russians to refrain from retaliation against Obama administration sanctions — hinting that Trump would soften them once in office.

WATCH: On Path to Reelection, Trump Takes Aim at Obama

Oval Office meeting 

Central to the Obamagate conspiracy theory is an Oval Office meeting between Obama and his national security team on Jan. 5, 2017, just 15 days before Trump took office.

The meeting came the day after the FBI moved to formally close its investigation into Flynn's suspected ties to Russia before deciding to keep it open after learning about his intercepted calls to Kislyak.

Top intelligence officials briefed Obama on their findings on Russian election meddling. At the end of the meeting, the president asked FBI Director James Comey and Deputy Attorney General Sally Yates to stay behind. Joining them were Vice President Joe Biden and National Security Adviser Susan Rice. 

FILE - Deputy Attorney General Sally Yates speaks during a news conference at the Justice Department in Washington, June 28, 2016.

As Yates recounted during a 2017 interview with special counsel Robert Mueller's team, Obama began by saying "he had learned of the information about Flynn" and his conversations with Kislyak regarding the sanctions on Russia.

This was news to Yates, who as the Justice Department's No. 2 official oversaw the FBI but had not been told about it.

As Comey later told the House Intelligence Committee, he alerted CIA Director John Brennan as soon as he learned about the Flynn calls. Brennan in turn briefed Obama.

Yates and Rice later recounted what Obama said at the meeting. According to Yates, while Obama said he did not want details of the investigation, he asked "whether the White House should be treating Flynn differently" during the remaining days of the administration.

FILE - FBI Director James Comey testifies on Capitol Hill in Washington, Sept. 27, 2016.

Yates did not recall Comey's response to the question. In an email to herself on Obama's last day in office, Rice memorialized that Obama reiterated at the meeting that "our law enforcement team needs to proceed as it normally would by the book."

Yates and other former officials have defended the Flynn investigation. Yet, the fact that the White House meeting came the day after the FBI was preparing to close the Flynn investigation, and Obama was aware of Flynn's wiretapped calls to Kislyak, has led Trump, right-wing commentators and Flynn's lawyers to claim that the subsequent Flynn investigation over his calls to Kislyak constituted an anti-Trump conspiracy that reached the highest levels of the Obama administration.

"So, the whole thing was orchestrated and set up within the FBI, (former national intelligence director James) Clapper, Brennan and in the Oval Office meeting that day with President Obama," Sydney Powell, Flynn's attorney, told Fox News on Sunday.

Recently declassified FBI records of the Flynn case that were turned over to Flynn's lawyers provided further fodder for conspiracy theory promoters.

In one widely cited handwritten note, a top FBI official mused whether the goal of interviewing Flynn over his calls to the Russian ambassador was to "get him fired" or get him to lie.

Unmasking of Flynn

The Obamagate conspiracy theory feeds off another allegation: the politically motivated unmasking of Flynn's identity during the Trump transition period.

U.S. intelligence analysts routinely "mask" the identity of U.S. persons whose communications are incidentally collected during intelligence-gathering on foreign officials. Flynn's conversations with Kislyak were apparently overheard during a routine interception of Kislyak's calls.

Authorized national security officials seeking to understand the underlying intelligence can ask the National Security Agency to "unmask" an individual's identity.

This is a common practice. But Trump and his allies have long accused former Obama administration officials of illegally unmasking Flynn's identity for political purposes.

This week, Richard Grennell, a close Trump ally who serves as acting director of national intelligence, disclosed the names of more than a dozen Obama officials who requested the unmasking of Flynn's identity during the final weeks of the administration.

The list includes a string of Obama administration officials Trump has long viewed as his enemies — Brennan, James Clapper, director of national intelligence, and Comey.

Prospects for an investigation

Since starting Obamagate, Trump has been pressing congressional Republicans to investigate Obama. That prospect is highly unlikely.

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

FILE - Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., speaks to reporters on Capitol Hill, Feb. 25, 2020.

But Senate Judiciary Chairman Lindsey Graham, a Trump confidant, says he is not interested in dragging Obama before Congress.

"I don't think now's the time for me to do that," Graham told Politico. "I don't know if that's even possible."

But Graham vowed to call in Trump administration officials as his committee investigates the origins of the Trump-Russia investigation.

For their part, Biden and other Democrats dismiss Obamagate as a blatant attempt by Trump to divert attention from growing criticism of his handling of the coronavirus crisis.