President Donald Trump broke with tradition Thursday by calling for investigation of his White House predecessor Barack Obama in the latest attempt to push a conspiracy theory about his Democratic opponents.

Washington operates under unwritten rules that presidents and ex-presidents avoid public clashes.

However Trump, less than six months from a volatile reelection showdown, called for Obama to testify in the Senate over a conspiracy theory he has dubbed "Obamagate."

The claim is that Obama's administration and a "deep state" tried to scuttle Trump's presidency using probes into his contacts with Russian entities.

"Just do it," Trump tweeted, addressing Senator Lindsey Graham, one of his most loyal Republican allies, who chairs the powerful Senate judiciary committee.

"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 said in the tweet.

Shortly after, Obama appeared to respond on Twitter, writing simply: "vote."

And Graham poured cold water on Trump's idea, telling Politico: "I don’t think now's the time for me to do that. I don't know if that's even possible."

However Trump is set to keep pushing his "Obamagate" agenda as he works overtime to stir enthusiasm in his right-wing base ahead of the November 3 presidential election.

Among the figures he is now targeting is Obama's vice president Joe Biden, the presumptive Democratic challenger this November.

The conspiracy theory seeks to turn on its head a two-year investigation led by special counsel Robert Mueller into Trump's Russia contacts.

Mueller determined that Trump and his election campaign had extensive, sometimes murky contacts with Russians, and that Moscow directly interfered in the tense 2016 election.

But he said there was no hard evidence of collusion between the two sides.

Trump has called for punishing the probe's backers, calling the entire affair a "hoax." 

Despite saying that he was "greatly concerned by the precedent that would be set by calling a former president" to testify, Graham promised to hold hearings in June on the Russia-Trump investigation in general.

https://www.youtube.com/watch?v=pOavNaBX4NU