Aides said the episode reflected how the persistent questions about Mr. Trump’s ties to Russia have all but paralyzed the president. Baffled by his solicitous tone toward Mr. Putin, some people close to Mr. Trump have concluded that he feels vulnerable to Mr. Putin, even if it is in his own mind, rather than because of any damaging information possessed by the Russians.

Even as he walked back his remarks, Mr. Trump repeated his assertion that there was no evidence of collusion between his campaign and the Russians. That line was scribbled in black marker onto a typewritten sheet of remarks on the table before him.

And he also seemed to undercut his own assertion that he had accepted the findings of Russian involvement.

“Could be other people also,” Mr. Trump said, appearing to ad-lib. “A lot of people out there.”

That echoed his previous musings that the interference could have been carried out by China, a guy from New Jersey or “somebody sitting on their bed who weighs 400 pounds.”

As in other times Mr. Trump has ignited a furor — notably after his equivocal reaction to the violent attacks by white supremacists on left-wing activists in Charlottesville, Va. — his demeanor was at odds with the message he was ostensibly delivering.

At one point during his remarks, the TV lights in the Cabinet Room, where Mr. Trump was meeting with lawmakers, switched off, plunging the room into gloomy shadows. “Whoops, they just turned off the lights,” Mr. Trump joked. “That must be the intelligence agencies.”

Barely 24 hours earlier, the president had stood next to Mr. Putin under the glittering chandeliers of a ballroom in Helsinki, telling aides after the news conference wrapped up that he was happy with how it had gone. In an interview afterward with the Fox News host Sean Hannity, the president was upbeat, describing the meeting with Mr. Putin that preceded it as productive and speaking of a new era of cooperation with Russia.