US president Donald Trump openly questioned his own intelligence agencies' finding that Russia meddled in the 2016 US election, seeming to accept Vladimir Putin's insistence that Moscow's hands were clean.
The reaction in the United States was immediate and visceral among fellow Republicans and usual Trump critics.
"Shameful," "disgraceful," and "weak" were a few of the comments, while senator Bob Corker said it makes the US "look like a pushover".
In an interview with Fox News' Sean Hannity broadcast on Monday evening, Mr Trump said "it's a shame" that he and Putin were being asked questions about the Russia probe while they were trying to discuss issues like Syria and nuclear proliferation.
"We've had a phony witch hunt deal drive us apart," he said.
Standing alongside Mr Putin in Helsinki, Mr Trump steered clear of any confrontation with the Russian, going so far as to question American intelligence and last week's federal indictments that accused 12 Russians of hacking into Democratic email accounts to hurt Hillary Clinton in 2016.
"I have great confidence in my intelligence people, but I will tell you that president Putin was extremely strong and powerful in his denial today.
"He just said it's not Russia. I will say this: I don't see any reason why it would be," Mr Trump said.
His scepticism drew a quick formal statement - almost a rebuttal - from Mr Trump's director of national intelligence, Dan Coats.
"We have been clear in our assessments of Russian meddling in the 2016 election and their ongoing, pervasive efforts to undermine our democracy, and we will continue to provide unvarnished and objective intelligence in support of our national security," Mr Coats said.
Listen to US political commentator Alan Friedman talking to Julia Hartley-Brewer on talkRADIO.
Fellow Republican politicians have generally stuck with Mr Trump during a year and a half of turmoil, but he was assailed as seldom before as he returned home on Monday night from what he had hoped would by a proud summit with Putin.
Senator John McCain of Arizona was most outspoken, declaring that Mr Trump made a "conscious choice to defend a tyrant" and achieved "one of the most disgraceful performances by an American president in memory".
House Speaker Paul Ryan, who rarely criticises Mr Trump, stressed there was "no question" that Russia had interfered.
Even staunch Mr Trump backer Newt Gingrich, the former House speaker, called the president's comments "the most serious mistake of his presidency" and said they "must be corrected - immediately".
As he flew home to Washington aboard Air Force One, Mr Trump tried to clarify his position via tweet, saying: "As I said today and many times before, 'I have GREAT confidence in MY intelligence people.' However, I also recognise that in order to build a brighter future, we cannot exclusively focus on the past - as the world's two largest nuclear powers, we must get along!"
The two leaders' long-awaited summit began with a private face-to-face sitdown - just the leaders and their interpreters - that lasted more than two hours, before additional meetings joined by senior aides.