During their presentation at Mr Trump’s impeachment trial on Wednesday, the House impeachment managers argued that the former president refusing to denounce the Proud Boys helped lead to the siege at the US Capitol on 6 January.
At the first presidential debate on 30 September 2020, Mr Trump told the group in widely criticised remarks to “stand back and stand by” when asked to denounce them by the moderator Chris Wallace.
Multiple Proud Boys members were seen at the riots on 6 January, as at least seven of them have been arrested in the weeks since, for allegedly entering the Capitol.
Five people died and several more were injured in the insurrection. A week later, and a week before leaving office, Mr Trump was impeached by the House for the second time for inciting the riots at a “Save America” rally nearby.
Speaking to reporters after the second day of the impeachment trial on Wednesday, Mr Graham, a senator from South Carolina, criticised the Democrats for allegedly attempting to tie Mr Trump to the far-right group.
“The legal theory they have is absurd – that somehow that Trump’s a secret member of the Proud Boys,” he said.
Later in the day during an appearance on Fox News’ Hannity, Mr Graham reiterated his claims, calling the argument “Looney Tunes”.
He then claimed: "The managers have got this cockamamie idea, an absurd theory that Donald Trump was monitoring the Proud Boys website and other far-right websites and he and [former White House Deputy Chief of Staff] Dan Scavino knew this was going to happen and they encouraged it.”
Proud Boys is a far-right, male-only organisation known for engaging in violent clashes at political rallies, which has been described as an extremist group by the legal advocacy organisation, the Southern Poverty Law Centre.
Facebook and Instagram have also banned content from members of the group from appearing on their sites, with the former explaining its decision by saying that it bans “hate organisations” from its service.
Jonathan Zucker, the lawyer of Dominic Pezzola, one of the Proud Boys arrested in connection to the riots, blamed Mr Trump for the events of 6 January in a court filing on Wednesday, claiming that his client was “was one of millions of Americans who were misled by the President’s deception.”
He added that “many of those who heeded his call will be spending substantial portions if not the remainder of their lives in prison as a consequence. Meanwhile Donald Trump resumes his life of luxury and privilege.”
Mr Graham also claimed that “the 'Not Guilty' vote is growing after today,” hours after the impeachment managers showed never-before-seen footage from the violent riot at the Capitol.
“I think most Republicans found the presentation by the House Managers offensive and absurd,” he wrote in a tweet on Wednesday, but added on Hannity later in the day: “We all know what happened in the Capitol was terrible. I hope everybody involved that broke into the Capitol goes to jail.”
Mr Trump’s Senate impeachment trial began on 9 February, with it expected to continue for around a couple of weeks. If he is convicted, then Mr Trump could be barred from running for office again.
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