Donald Trump has called for Barack Obama to be hauled before Congress, and for his predecessor’s officials to be “jailed for 50 years”.
He claims that, during the presidential transition period at the end of 2016, the Obama administration abused its power for political reasons in targeting Mr Flynn.
He told Fox Business that senior Obama officials should “have been in jail a long time ago, and I’m talking with 50-year sentences. It is a disgrace what’s happened. This is the greatest political scam, hoax in the history of our country.”
The president added: “People should be going to jail for this stuff and hopefully, a lot of people are going to have to pay.”
Mr Obama responded by tweeting: “Vote.”
Mr Trump also urged Lindsey Graham, the Republican chairman of the Senate judiciary committee, with whom he plays golf, to call Mr Obama to give evidence.
He told Fox Business: “The president knew everything. President Obama and Vice President Biden, they knew everything.”
Names of Americans picked up in US surveillance of foreign individuals are routinely hidden in intelligence reports.
US officials must make a specific request if they want to know the American person’s identity, a process known as “unmasking”.
A list of the Obama officials who asked for the unmasking of an individual, who turned out to be Mr Flynn, was recently declassified by Richard Grenell, Mr Trump’s acting national intelligence director.
The list included Joe Biden, who was Mr Obama’s vice president, the FBI Director James Comey, and Samantha Power, the former US ambassador to the United Nations.
“He was one of the unmaskers,” Mr Trump said, referring to Mr Biden.
Mr Flynn had been picked up in surveillance of the Russian ambassador,and was later prosecuted for lying to the FBI over his connections to the ambassador. He pleaded guilty.
In recent days the justice department dropped the case against Mr Flynn.
Mr Obama said that decision showed the “rule of law is at risk”.
Supporters of Mr Trump have embraced the “Obamagate” theory as a rallying cry.
Retired Admiral Ronny Jackson, who served as White House doctor for both Mr Obama and Mr Trump,said: “President Obama weaponised the highest levels of our government to spy on President Trump.
“Every Deep State traitor deserves to be brought to justice for their heinous actions.”
A spokesman for the Biden campaign said: “All normal procedures were followed. Any suggestion otherwise is a flat out lie.”
Mr Biden and the other Obama administration officials had the authority to seek the unmasking of an unidentified American in intelligence reports, and it turned out to be Mr Flynn.
Critics have accused Mr Trump of trying to distract from the coronavirus crisis, and spiraling unemployment.
But Mr Trump wrote on Twitter: “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. No more Mr. Nice Guy. No more talk!”
Mr Graham said he would not be calling Mr Obama.
He said: “I don’t think now’s the time for me to do that. I don’t know if that’s even possible.I understand President Trump’s frustration, but be careful what you wish for.”
Mr Graham said both Mr Trump and Mr Obama were “welcome to come before the committee and share their concerns about each other. If nothing else it would make for great TV.”
Mr Biden said on Thursday night that if he wins the presidency, he would not use his power to pardon Mr Trump or stop any investigations of the president and his associates.
“It is not something the president is entitled to do, to direct a prosecution or decide to drop a case,” Mr Biden said on MSNBC. “It’s a dereliction of duty.”
The former vice-president was responding to a voter who asked him on Lawrence O’Donnell’s show, “The Last Word,” whether Biden would “commit to not pulling a Gerald Ford in giving Donald Trump a pardon under the pretense of healing the nation.” Biden responded, “I commit,” before saying that the president must allow the Justice Department to operate without interference.
Ford became president in 1974 when Richard Nixon resigned under the threat of impeachment. Ford later pardoned his predecessor before any criminal charges related to the Watergate burglary could be filed.