Russian hackers tried to penetrate the computer networks of the Republican National Committee, using the same techniques that allowed them to infiltrate its Democratic counterpart, according to U.S. officials who have been briefed on the attempted intrusion.

But the intruders failed to get past security defenses on the RNC’s computer networks, the officials said. And people close to the investigation said it indicated a less aggressive and much less persistent effort by Russian intelligence to hack the Republican group than the Democratic National Committee. Only a single email account linked to a long-departed RNC staffer was targeted.

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The disclosures came as a political furor grows over suspected Russian hacking of U.S. political organizations. The Central Intelligence Agency has concluded that Russian hackers, whom analysts say work for that country’s military and intelligence apparatus, stole emails from the DNC, as well as another Democratic organization and the chairman of Hillary Clinton’s presidential campaign, to harm her candidacy and boost Republican Donald Trump’s chances of winning. Russia has denied the allegations.

The possibility that Russians tried and failed to infiltrate the RNC doesn’t necessarily conflict with the CIA’s conclusion. A senior U.S. official said analysts now believe what started as an information-gathering campaign aimed at both parties later took on a focus of leaked emails about Mrs. Clinton and Democrats.

Mr. Trump has discounted the U.S. intelligence assessments and disparaged intelligence officials. In a Twitter message Thursday, he wrote: “If Russia, or some other entity, was hacking, why did the White House wait so long to act? Why did they only complain after Hillary lost?”

The Obama administration warned for months that Russian hackers had tried to interfere with U.S. elections, and intelligence agencies issued an unusual public assessment in October warning Russia was behind the cyberattack.

In an NPR interview Thursday, President Barack Obama said the U.S. is considering retaliatory moves against Russia. “I think there is no doubt that when any foreign government tries to impact the integrity of our elections, that we need to take action, and we will,” he said. “At a time and a place of our own choosing. Some of it may be explicit and publicized; some of it may not be.”

Until now, few details had been disclosed about the nature of the targeting of Republican organizations, especially the flagship Republican National Committee, where hackers sent so-called phishing emails last spring to an email address there. Those emails were quarantined by a filter meant to detect spam as well as potentially malicious traffic that may carry viruses or trick recipients into divulging passwords, two officials said.

A third person familiar with the investigation said RNC staff members didn’t realize they had been the target of spies until June, after Democratic committee leaders revealed that hackers had successfully gained a foothold inside their networks. Once inside, they reportedly were able to access a trove of DNC opposition research on Mr. Trump, then a candidate.

Donald Trump signaled during the campaign that he would take a more cordial stance towards Russia. However, allegations of hacking during the election may hamper negotiations on issues including Ukraine, Syria and arms control. Photo: AP

RNC officials, concerned they too might have been compromised, called a private computer security firm, which in turn called the FBI and obtained information about what kinds of malicious emails to look for, the person said. Upon inspection, the RNC found that its electronic filters had blocked emails sent to a former employee matching the description they’d been warned about.

The apparently successful blocking of a Russian espionage operation offers one possible explanation why the GOP’s main political organization didn’t suffer the same fate as its Democratic counterpart—a deluge of leaked emails revealing private correspondence and internal strategy.

But the suspicion that Russians did try to break into the RNC, using the same techniques and tactics that worked so well on the Democrats, suggests that at least initially, they were trying to gather potentially incriminating or embarrassing information on both parties.

Hackers who overran the DNC also successfully stole and then published emails from Republican state-level officials on a website, DCLeaks, that private security experts have linked to the Russian operation allegedly targeting the DNC and the presidential campaign of Mrs. Clinton. The leaked GOP emails received relatively little public attention and consisted of mostly mundane and innocuous correspondence, including inquiries and complaints sent by constituents to generic addresses.

On Wednesday, Republican Sen. Lindsey Graham of South Carolina told CNN his campaign email system was hacked by Russian operators. Mr. Graham plans to hold hearings about the hacks, which all U.S. intelligence agencies said in a rare public statement in October were directed by the highest-levels of the Russian government to interfere with U.S. elections.

Democratic presidential candidate Hillary Clinton is seen on a television at the Republican National Committee in Washington on Nov. 9, before Donald Trump spoke about the GOP's success on election night. Photo: Alex Brandon/Associated Press

Leaked emails revealed that DNC staffers, who had publicly not pledged support for one candidate, were working behind the scenes to undermine Mrs. Clinton’s primary rival, Vermont Sen. Bernie Sanders.

Those messages were conveyed to WikiLeaks, where they attracted far greater attention than any of the GOP emails posted on DCLeaks, and handed Russian President Vladimir Putin a public-relations victory.

“If you look at the leaks that came out most recently on the DNC, the Russian media were very, very quick to pick up the fact that, ‘See, the West is always lecturing us, and yet look at this primary system. Was it a truly level playing field?’ Which is something the Americans always criticize the Russians about,” Peter Clement, a deputy assistant director at the CIA and the agency’s most senior Russia analyst, said at a conference in Washington in September.

Other emails raised questions about whether Mrs. Clinton used her position as secretary of state to do favors for donors to her family’s foundation.

President Barack Obama said the U.S. will take action against Russia for its alleged involvement in the U.S. presidential election, after intelligence officials concluded that Russian President Vladimir Putin personally authorized the hacking of institutions and members of the Democratic party. Photo: AP.

Independent computer security experts said it was plausible that the RNC’s filtering systems stopped any potentially dangerous emails from reaching their targets. But the fact that they failed doesn’t mean they preferred one candidate over another, nor that they don’t harbor plans to leak embarrassing information about Republicans or Mr. Trump in the future, now that he’s about to assume the presidency.

“It would be naive [for Republicans] to think they weren’t targeted,” said Michael Buratowski, a senior vice president at Fidelis Cybersecurity, which examined the DNC breach and attributed it to the Russian hackers.

“It doesn’t surprise me at all that they were gone after,” Mr. Buratowski said, because the hackers in question have demonstrated over many years a voracious appetite for information. The two groups have been linked to earlier incursions at U.S. government agencies, including the State Department and the White House.

—Damian Paletta contributed to this article.

Write to Devlin Barrett at devlin.barrett@wsj.com, Shane Harris at shane.harris@wsj.com and Julian E. Barnes at julian.barnes@wsj.com