Washington (CNN) President Donald Trump on Wednesday morning again labeled the probe into potential coordination between his associates and Russia the "single greatest witch hunt" in US history.

"The single greatest Witch Hunt in American history continues. There was no collusion, everybody including the Dems knows there was no collusion, & yet on and on it goes. Russia & the world is laughing at the stupidity they are witnessing. Republicans should finally take control!" Trump tweeted.

The single greatest Witch Hunt in American history continues. There was no collusion, everybody including the Dems knows there was no collusion, & yet on and on it goes. Russia & the world is laughing at the stupidity they are witnessing. Republicans should finally take control!

It's a familiar charge from Trump. Whether in public remarks or on Twitter, the President has repeatedly called the special counsel investigation led by former FBI Director Robert Mueller a "witch hunt," dismissing it as a frivolous investigation launched by his political enemies seeking to delegitimize his 2016 election victory.

It's true that Mueller's probe hasn't yet turned up any evidence of collusion between Trump's campaign and the Russian government, or at least hasn't released any such evidence publicly. But his investigation has already resulted in Trump's former national security adviser, Michael Flynn, pleading guilty to lying to the FBI about conversations with Russia's ambassador. And it's been revealed in the press that Donald Trump Jr. eagerly accepted a meeting in June 2016 on the premise that he would be offered damaging information about Hillary Clinton from a "Russian government attorney."

Shortly after CNN published a bombshell report revealing that top US intelligence figures presented Trump with claims of Russian efforts to compromise him, Trump tweeted on January 10, 2017: "FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!"

He said at the end of March that Flynn should ask for immunity due to the "witch hunt" by the media and the Democratic Party. Later that year, however, Flynn pleaded guilty and said he would cooperate with Mueller.

Mike Flynn should ask for immunity in that this is a witch hunt (excuse for big election loss), by media & Dems, of historic proportion! — Donald J. Trump (@realDonaldTrump) March 31, 2017

Trump, speaking alongside Colombian President Juan Manuel Santos in May after he fired James Comey as FBI director, reacted to the appointment of a special counsel by saying, "Well, I respect the move but the entire thing has been a witch hunt."

In June, Trump said he was "being investigated" for firing Comey, and again called it a "witch hunt." Before and since the tweet, Trump and his lawyers have said the President is not under investigation.

I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt — Donald J. Trump (@realDonaldTrump) June 16, 2017

Speaking aboard Air Force One in July, Trump used the phrase "witch hunt" several times, declaring ongoing questions about Russia to be a politically motivated "witch hunt" overplayed by the press.

Trump last July praised his son Donald Trump Jr. and son-in-law, Jared Kushner, for their actions in face of the "witch hunt." On his tweet praising Kushner, Trump joked that the next one facing scrutiny over ties to Russia would be his youngest son, Barron Trump.

My son Donald did a good job last night. He was open, transparent and innocent. This is the greatest Witch Hunt in political history. Sad! — Donald J. Trump (@realDonaldTrump) July 12, 2017

Jared Kushner did very well yesterday in proving he did not collude with the Russians. Witch Hunt. Next up, 11 year old Barron Trump! — Donald J. Trump (@realDonaldTrump) July 25, 2017

Then-FBI Director Comey revealed the existence of the investigation during a House hearing last March, and after Comey's firing, Deputy Attorney General Rod Rosenstein appointed Mueller to lead a special counsel investigation.

Mueller's appointment enjoyed bipartisan support, although as the probe has continued, more Trump allies have joined the President in trying to discredit it.

The term "witch hunt" often evokes such dark chapters in US history as the anti-communist hysteria of the 1950s or the literal witch hunts in 17th-century Salem, Massachusetts.