"We believe, based on the scope and sensitivity of these efforts, that only Russia's senior-most officials could have authorized these activities," a US government statement said on Friday about hacking of political groups.
"These thefts and disclosures are intended to interfere with the US election process," the statement said.
US intelligence officials concluded weeks ago that the Russian government was conducting or orchestrating cyber attacks against the Democratic National Committee and the Democratic Congressional Campaign Committee, possibly to disrupt or discredit the election, in which Democrat Hillary Clinton faces Republican Donald Trump.
The Obama administration's decision to blame Russia for the attacks is the latest downward turn in Washington's relations with Moscow, which are under strain over Russia's actions in Syria and Ukraine and in cyberspace.
Also on Friday, US Secretary of State John Kerry said Russian and Syrian actions in the Syrian civil war, including bombings of hospitals, "beg for" a war crimes investigation.
In addition, a US intelligence official said on Friday that Russia is moving short-range nuclear-capable missiles into Kaliningrad, a tiny Russian enclave between Poland and Lithuania, confirming Estonian news reports.
Clinton's campaign chairman, John Podesta, said public blaming for the hacks left one remaining question of "why Donald Trump continues to make apologies for the Russians." Trump had previously expressed doubt about Russia's involvement. In July, he suggested Russia should attempt to retrieve and publish emails from Clinton's private server.
Trump's campaign did not immediately respond to a request for comment.
Hours after the US government's accusation was levied, WikiLeaks posted hundreds of emails on its website purportedly hacked from Podesta's private account.
The statement by the Department of Homeland Security and the Office of the Director of National Intelligence did not blame the Russian government for hacking attempts against state election systems, but said "scanning and probing" of those systems originated in most cases from servers operated by a Russian company.
However, a Department of Homeland Security spokesman told Reuters that US officials have concluded that the hacking attacks or probes of state voter registration systems are "consistent with Russian motivations."
Concern has grown about the reliability of the US voting system as a result of the breach, and Republican presidential candidate Donald Trump has called the system "rigged," but without providing specific evidence.
US intelligence officials have said there is no evidence that voting recording systems have been manipulated.
Naming Russia as the actor behind the cyber attacks on political organizations falls short of more punitive measures the United States has taken against other countries for cyber intrusions.
Lawmakers of both political parties welcomed the formal accusation. Republican Senator Cory Gardner, chairman of the U.S. Senate Foreign Relations Subcommittee on East Asia, the Pacific and International Cybersecurity, said he planned to introduce sanctions legislation.