Few Democrats believe that Mr. Trump won solely as a result of Russia’s intervention. But many think the meddling exacerbated Mrs. Clinton’s challenges, making her more vulnerable to what some believe was the decisive blow: the announcement by James B. Comey, the F.B.I. director at the time, just over a week before the election that he was reopening the investigation into her use of a private email server.

“Russia succeeded in weakening her enough so that the Comey letter could knock her off,” said Jennifer Palmieri, who was Mrs. Clinton’s campaign communications director.

But veterans of Mr. Trump’s campaign shake their head at what they believe is an abiding sense of denial among Clinton loyalists over her deficiencies.

“The election was still won because Donald Trump was a better candidate with a superior message taking on a horrible candidate who people thought was corrupt,” said David Bossie, a senior Trump strategist who is now president of the conservative lobbying group Citizens United. He allowed that Moscow had mounted a “malicious, disruptive campaign.”