But Mr. Rubio immediately sought to play down his remarks, cautioning his colleagues during the hearing not to become too focused on hacking, “one aspect of a much broader campaign.”

“We’ve focused on the trees and have lost sight of the forest,” Mr. Rubio said.

Other Republicans on the panel expressed some skepticism over the pervasiveness of Russian interference in the campaign. Senator Roy Blunt of Missouri asked whether the experts who had been brought in to testify had seen any evidence of direct Russian interference in county election systems, something he said “should never be allowed to happen.” No, they said.

Senator John Cornyn of Texas, the Senate’s No. 2 Republican, cast doubt on the conclusion of intelligence agencies that Russians had acted specifically to aid Mr. Trump in the election. “Do you have any reason to believe that Putin knew more than the pundits and pollsters did here in America about the outcome of the election before it occurred?” Mr. Cornyn asked, referring to the Russian president, Vladimir V. Putin.

No, came the reply.

“I didn’t think so,” Mr. Cornyn said.

In one remarkable exchange, Senator James Lankford, Republican of Oklahoma, asked why Mr. Putin had chosen this election to meddle in.

Clint Watts, a former F.B.I. agent who is now a senior fellow at the Foreign Policy Research Institute, said that while Russia had sought to interfere in elections around the world, its efforts in last year’s American election were especially potent because Mr. Trump’s campaign had “parroted” the same lines used in Russian propaganda. Mr. Trump cited a fake news article in at least one stump speech, Mr. Watts said, adding that his claims of a “rigged” election were in line with claims made by the Russian state news media.