Steve Bannon, a right-wing populist with a huge following, has been accused of making a Nazi salute while addressing the annual US Conservative Political Action Conference.
It is the second time such a gesture has been seen on a major Republican stage, after Elon Musk made a similar sign following President Donald Trump's inauguration in January.
Bannon, who served as a top adviser to Mr Trump during his first term, thrust his arm into the air with his hand facing down as he delivered a fiery speech at the (CPAC) event just outside Washington late on Thursday.
“The hardest toughest days are ahead, folks … we're not going to retreat, we're not going to surrender, fight, fight, fight,” he exclaimed as he gestured, prompting cheers and whoops from the audience.
The gesture prompted a significant backlash online from groups such as Republicans Against Trump. Jordan Bardella, the president of France’s right-wing National Rally party, cancelled his scheduled speech at CPAC in response.
Mr Bardella accused Bannon, who has pleaded guilty to fraud in a New York case and spent four months in prison last year after being convicted of contempt of Congress, making “a gesture alluding to Nazi ideology”.
Bannon was also cheered when he said Mr Trump should defy the US Constitution's two-term presidency limit and described his former boss as “an instrument of divine providence”.
“We want Trump in '28, that's what they can't stand. A man like Trump comes along only once or twice in a country's history,” he added.
Mr Trump has made significant progress in swearing in an increasingly loyalist cabinet, working with Mr Musk to deliver on his promise to purge the federal workforce. He also is issuing a sweeping series of executive orders to consolidate his power over government agencies.
Mr Musk, the world's richest man whom Mr Trump appointed to lead his Department of Government Efficiency, or Doge, made the same gesture twice during his speech at a Washington rally following the January 20 inauguration. Mr Musk has accused Democrats of “dirty tricks” though he has not outright denied accusations that he made a fascist salute.
The salutes from Bannon and Mr Musk unite the figureheads of two warring factions within Mr Trump's “Make America Great Again”, or Maga, movement.
Bannon has led the anti-Musk wing in the feud over the role of tech billionaires in the new administration, a dispute that has been dubbed the “Maga civil war”.
He has accused Mr Musk and the surge of Silicon Valley big-tech support of the Maga movement of being “oligarchs” who are “complete atheistic 11-year-old boys … and we've turned the nation over to that, and I'm going to fight that every step of the way,” he recently told The New York Times' conservative columnist Ross Douthat.
Last year, John Kelly, a retired general and Mr Trump's White House chief of staff in his first term, said his old boss fits “the general definition of a fascist” and had said positive things about Adolf Hitler.