WASHINGTON — The Obama administration struck back at Russia on Thursday for its efforts to influence the 2016 election, ejecting 35 Russian intelligence operatives from the United States and imposing sanctions on Russia’s two leading intelligence services, including four top officers of the military intelligence unit the White House believes ordered the attacks on the Democratic National Committee and other political organizations.
In a sweeping set of announcements, the United States was also expected to release evidence linking the cyberattacks to computer systems used by Russian intelligence. Taken together, the actions would amount to the strongest American response ever taken to a state-sponsored cyberattack aimed at the United States.
The sanctions were also intended to box in President-elect Donald J. Trump. Mr. Trump has consistently cast doubt that the Russian government had anything to do with the hacking of the D.N.C. or other political institutions, saying American intelligence agencies could not be trusted and suggesting that the hacking could have been the work of a “400-pound guy” lying in his bed.
Mr. Trump will now have to decide whether to lift the sanctions on the Russian intelligence agencies when he takes office next month, with Republicans in Congress among those calling for a public investigation into Russia’s actions. Should Mr. Trump do so, it would require him to effectively reject the findings of his intelligence agencies.
Asked on Wednesday night at his Mar-a-Lago estate in Palm Beach, Fla., about reports of the impending sanctions, Mr. Trump said: “I think we ought to get on with our lives. I think that computers have complicated lives very greatly. The whole age of computer has made it where nobody knows exactly what is going on. We have speed, we have a lot of other things, but I’m not sure we have the kind, the security we need.”
The Obama administration is also planning to release a detailed “joint analytic report” from the Federal Bureau of Investigation and the Department of Homeland Security that is clearly based in part on intelligence gathered by the National Security Agency. A more detailed report on the intelligence, ordered by President Obama, will be published in the next three weeks, though much of the detail — especially evidence collected from “implants” in Russian computer systems, tapped conversations and spies — is expected to remain classified.
Despite the fanfare and political repercussions surrounding the announcement, it is not clear how much real effect the sanctions may have, although they go well beyond the modest sanctions imposed against North Korea for its attack on Sony Pictures Entertainment two years ago.
Starting in March 2014, the United States and its Western allies levied sanctions against broad sectors of the Russian economy and blacklisted dozens of people, some of them close friends of President Vladimir V. Putin, after the Russian annexation of Crimea and its activities to destabilize Ukraine. Mr. Trump suggested in an interview with The New York Times earlier this year that he believed those sanctions were useless, and left open the possibility he might lift them.
Mr. Obama and his staff have debated for months when and how to impose what they call “proportionate” sanctions for the remarkable set of events that took place during the election, as well as how much of them to announce publicly. Several officials, including Vice President Joseph R. Biden Jr., have suggested that there may also be a covert response, one that would be obvious to Mr. Putin but not to the public.
While that may prove satisfying, many outside experts have said that unless the public response is strong enough to impose a real cost on Mr. Putin, his government and his vast intelligence apparatus, it might not deter further activity.
“They are concerned about controlling retaliation,” said James A. Lewis, a cyberexpert at the Center for Strategic and International Studies in Washington.
The Obama administration was riven by an internal debate about how much of its evidence to make public. Although the announcement risks revealing sources and methods, it was the best way, some officials inside the administration argued, to make clear to a raft of other nations — including China, Iran and North Korea — that their activities can be tracked and exposed.
In the end, Mr. Obama decided to expand an executive order that he issued in April 2015, after the Sony hacking. He signed it in Hawaii on Thursday morning, specifically giving himself and his successor the authority to issue travel bans and asset freezes on those who “tamper with, alter, or cause a misappropriation of information, with a purpose or effect of interfering with or undermining election processes or institutions.”
Mr. Obama used that order to immediately impose sanctions on four Russian intelligence officials: Igor Valentinovich Korobov, the current chief of a military intelligence agency, the G.R.U., and three deputies: Sergey Aleksandrovich Gizunov, the deputy chief of the G.R.U.; Igor Olegovich Kostyukov, a first deputy chief, and Vladimir Stepanovich Alekseyev, also a first deputy chief of the G.R.U.
But G.R.U. officials rarely travel to the United States, or keep their assets here, so the effects may be largely symbolic. It is also unclear if any American allies will impose parallel sanctions on Russia.
The administration also put sanctions on three companies and organizations that it said supported the hacking operations: the Special Technologies Center, a signals intelligence operation in St. Petersburg; a firm called Zor Security that is also known as Esage Lab; and the Autonomous Non-commercial Organization Professional Association of Designers of Data Processing Systems, whose lengthy name, American officials said, was cover for a group that provided special training for the hacking.
“It is hard to do business around the world when you are named like this,” a senior administration official with long experience in Russia sanctions said on Thursday morning. The official spoke on the condition of anonymity because of the sensitive nature of the intelligence.
But the question will remain whether the United States acted too slowly — and then, perhaps, with not enough force. Members of Hillary Clinton’s election campaign argue that the distractions caused by the leaks of emails, showing infighting in the D.N.C., and later the private communications of John D. Podesta, the campaign chairman, absorbed an American press corps more interested in the leaks than in the phenomena of a foreign power marrying new cybertechniques with old-style information warfare.
Certainly the United States had early notice. The F.B.I. first informed the D.N.C. that it saw evidence that the committee’s email systems had been hacked in the fall of 2015. Months of fumbling and slow responses followed. Mr. Obama said at a new conference he was first notified early this summer. But one of his top cyberaides met Russian officials in Geneva to complain about cyberactivity in April.
By the time the leadership of the D.N.C. woke up to what was happening, the G.R.U. had not only obtained those emails through a hacking group that has been closely associated with it for years, but, investigators say, also allowed them to be published on a number of websites, from a newly created one called DC Leaks to the far more established WikiLeaks. Meanwhile, several states reported the “scanning” of their voter databases — which American intelligence agencies also attributed to Russian hackers. But there is no evidence, American officials said, that Russia sought to manipulate votes or voter rolls on Nov. 8.
Mr. Obama decided not to issue sanctions ahead of the elections, for fear of Russian retaliation ahead of Election Day. Some of his aides now believe that was a mistake. But the president made clear before leaving for Hawaii that he planned to respond.
The question now is whether the response he has assembled will be more than just symbolic, deterring not only Russia but others who might attempt to influence future elections.