In recent days, a skeptical president-elect, the nation’s intelligence agencies, and the two major parties have become embroiled in an extraordinary public dispute over what evidence exists that President Vladimir V. Putin of Russia moved beyond mere espionage to deliberately try to subvert American democracy and pick the winner of the presidential election.

Many of Mrs. Clinton’s closest aides believe that the Russian assault had a profound impact on the election while conceding that other factors.  Mrs. Clinton’s weaknesses as a candidate; her private email server; the public statements of the F.B.I. director, James B. Comey, about her handling of classified information — were also important.

While there’s no way to be certain of the ultimate impact of the hack, this much is clear. A low-cost, high-impact weapon that Russia had test-fired in elections from Ukraine to Europe was trained in the United States, with devastating effectiveness. For Russia, with an enfeebled economy and a nuclear arsenal it cannot use short of all-out war. Cyber power proved the perfect weapon: cheap, hard to see coming, hard to trace.

The United States, too, has carried out cyber attacks, and in decades past the C.I.A. tried to subvert foreign elections. But the Russian attack is increasingly understood across the political spectrum as an ominous historic landmark — with one notable exception: Mr. Trump has rejected the findings of the intelligence agencies he will soon oversee as “ridiculous,” insisting that the hacker may be American, or Chinese, but that “they have no idea.”

Mr. Trump cited the reported disagreements between the agencies about whether Mr. Putin intended to help elect him. On Tuesday, a Russian government spokesman echoed Mr. Trump’s scorn.

“This tale of ‘hacks’ resembles a banal brawl between American security officials over spheres of influence,” Maria Zakharova, the spokeswoman for the Russian Foreign Ministry, wrote on Facebook.