But 13 months ago, Trump gave a far different explanation for the meeting. A July 2017 statement – dictated by the President and released on behalf of Donald Trump jnr – read: "We primarily discussed a program about the adoption of Russian children that was active and popular with American families years ago." Since then, the story about the meeting has changed several times, eventually forced by the discovery of emails between the President's eldest son and an intermediary from the Russian government offering damaging information about Trump's opponent, Hillary Clinton. Betraying no surprise or misgivings about the offer from a hostile foreign power, Trump jnr replied: "If it's what you say I love it especially later in the summer (sic)." Sunday's tweet was Trump's clearest statement yet in relation to the real purpose of the meeting, which has become a focal point of Mueller's investigation even as the President and his lawyers try to downplay its significance and pummel the investigation probe with attacks.

US President Donald Trump during a rally in Ohio on Saturday. Credit:AP On Sunday, Trump again suggested – without evidence – that Mueller was biased against him, declaring the probe "the most one sided witch hunt in the history of our country." As Trump and his allies have tried to discredit the probe, a new talking point has emerged: that even if that meeting was held to collect damaging information, none was provided and "collusion" – Trump's go-to description of what Mueller is investigating – never occurred. "The question is what law, statute or rule or regulation has been violated, and nobody has pointed to one," said Jay Sekulow, one of Trump's attorneys, on ABC's This Week. Divisive: Trump supporters clash with anti-Trump protesters outside a rally in Ohio on Saturday. Credit:Bloomberg

But legal experts have pointed out several possible criminal charges, including conspiracy against the United States and aiding and abetting a conspiracy. And despite Trump's public Twitter denial, the President has expressed worry that his son may face legal exposure even as he believes he did nothing wrong, according to three people close to the White House but not authorised to speak about private conversations. Sekulow acknowledged that the public explanation for the meeting has changed, but insisted that the White House had been very clear with Mueller's office. He said he was not aware of Trump jnr facing any legal exposure. "I don't represent Don jnr," Sekulow said, "but I will tell you I have no knowledge at all of Don jnr being told that he's a target of any investigation, and I have no knowledge of him being interviewed by the special counsel." Loading Trump's days of private anger spilled out into public with the Twitter outburst, which comes at a perilous time for the President.

A decision about whether he sits for an interview with Mueller may come in the coming weeks, according to another of his attorneys, Rudy Giuliani. Trump has seethed against what he feels are trumped-up charges against his former campaign chairman, Paul Manafort, whose trial began last week and provided a visible reminder of Mueller's work. And he raged against the media's obsession with his links to Russia and the status of Michael Cohen, his former fixer, who is under federal investigation in New York. Cohen has indicated that he would tell prosecutors that Trump knew about the Trump Tower meeting ahead of time. Despite a show of force from his national security team this week as a warning against future Russian election meddling, Trump again deemed the matter a "hoax" this week. At a trio of rallies, he escalated his already vitriolic rhetoric toward the media, savaging the press for what he claims is unflattering coverage. Loading