The Washington Post's Greg Miller explains what President-elect Donald Trump's clash with the CIA over Russia's suspected election interference means and how it started. (The Washington Post)

The CIA assessment that Russia waged a cyber-campaign to help elect Donald Trump is based in part on intelligence suggesting that Moscow’s hacking efforts were disproportionately aimed at targets tied to the Democratic Party and its nominee, Hillary Clinton, U.S. officials said.

U.S. officials said that both parties were repeatedly targeted as part of a months-long cyber-operation linked to Moscow, but that Democratic institutions and operatives came under a more sustained and determined online assault.

U.S. officials said the Republican National Committee’s computer systems were also probed and possibly penetrated by hackers tied to Russian intelligence services, but that it remains unclear how much material — if any — was taken from the RNC.

The lack of a corresponding Republican trove has contributed to the CIA assessment, reported by The Washington Post, that Russia was seeking to elect Trump and not merely to disrupt last month’s presidential election.

The disclosure of that CIA finding has roiled Washington, prompting calls for a broad congressional probe, compounding tensions between the president-elect and U.S. spy agencies, and extending the turmoil of a turbulent campaign.

CIA officials told senators it is now “quite clear” that electing Donald Trump was Russia’s goal. In an interview on Fox News Sunday on Dec. 11, President-elect Trump denied the CIA's assessment. (Victoria Walker/The Washington Post)

Clinton officials on Monday joined calls for U.S. intelligence agencies to share their findings with members of the electoral college before they formally cast their votes, a step that would represent an extraordinary departure from a process that is typically an overlooked formality in the U.S. election process.

Trump has denounced the CIA conclusion as “ridiculous.” Republican operatives have vehemently disputed that their party’s computer systems were hacked or, by implication, that Russia possessed but “sat” on a secret trove of emails or other materials that might have undermined the GOP nominee.

U.S. intelligence officials said that the Russian government appears to have succeeded in penetrating computer systems associated with both parties, but “prioritized” Democratic institutions in a campaign that culminated with the posting of thousands of sensitive emails on the WikiLeaks website.

The CIA briefed the administration that it thinks the Russians “breached” the RNC systems, according to a senior U.S. official, who like others spoke on the condition of anonymity because of the sensitivity of the issue. “Obviously there haven’t been the same sort of leaks and pilfered documents spread about from this intrusion as there were from the various DNC and related incursions.”

Other officials familiar with the CIA’s assessment said there is “high confidence” that the RNC was targeted but less certainty that the Russians got inside the committee and stole material.

The explanation for the lack of damaging Republican leaks remains a source of debate across the U.S. intelligence community, but officials think it may reflect in part an allocation of resources and effort by Russian-backed actors who have been tied to the hacking campaign.

President-elect Donald Trump as well as Republican and Democratic lawmakers on Dec. 11 reacted to the CIA’s assessment that Russia intervened to help Trump win the election. (Bastien Inzaurralde/The Washington Post)

There are other possible explanations still being explored, with some officials noting that Republican Party computers may simply have been better protected from intrusion.

That theory is viewed with skepticism, however, by security officials and experts who describe Russia as a highly capable cyber-adversary, and see it as unlikely that a concerted campaign orchestrated by Moscow would fail to extract material.

“If the Russians want to get into an unclassified network, one that has young staffers, it’s not going to be that hard,” said a U.S. official who has reviewed intelligence on the intrusions. The official said that the intelligence gathered so far indicates a more “sustained effort” was aimed at Democratic targets.

The White House and CIA officials declined to comment.

U.S. officials emphasized that intelligence agencies are continuing to gather and examine intelligence in an effort to reconstruct the alleged Russian intervention in a U.S. election, and that there are disagreements among agencies over how to interpret what they have assembled so far.

The FBI has been more cautious in assessing Russian intentions, and has so far stopped short of endorsing the CIA position that Russia “quite clearly” favored Trump. Some have attributed the competing positions to cultural differences between the bureau, accustomed to courtroom standards of proof, vs. the agency, whose mission is to help policymakers understand global events.

A senior FBI counterintelligence official was equivocal during a briefing with congressional officials last week — confirming that Republican systems had been targeted and acknowledging the apparent imbalance in damage done to Democrats, but refraining from assigning a pro-Trump motive to the Kremlin.

The RNC has vigorously disputed reports that its systems were hacked, let alone that Russia made off with a trove of material that it protected as part of an effort to elevate Trump’s election chances.

“The RNC was not ‘hacked,’ ” GOP spokesman Sean Spicer said in a statement posted to Twitter on Friday evening in response to a New York Times report. He said the report was “exhibit #1 in the fake news.”

U.S. officials said the competing claims may reflect disagreement over terminology. “There is no definition of ‘hack,’ ” a U.S. official said. “If you’re probed are you hacked? If a low-level staffer is breached are you hacked? Or do you have to get a massive release of documents?”

The DNC “was hacked by the common understanding of the word — the data was exfilled,” the official said, referring to the exfiltration of files. “Whether the data in the RNC was exfilled, maybe not. But if not, why not?”

Republicans contributed to the confusion surrounding the issue with contradictory statements before the election. Rep. Michael McCaul (R-Tex,) the chairman of the House Homeland Security Committee, said in September during a CNN interview that Russian operatives had “not only hacked into the DNC but also into the RNC.”

He subsequently said that he had misspoken, and that the targets had been “Republican political operatives.”

The material turned over to WikiLeaks included not just emails stolen from the DNC, but a separate trove taken from top Clinton campaign adviser John Podesta.

U.S. intelligence officials said the CIA has identified the “actors” who took possession of those stolen files and delivered them to WikiLeaks. The individuals are known for their affiliations to Russian intelligence services, but “one step” removed from the Russian government.

A senior administration official said that the CIA did not have “high confidence” about Russia’s motives until after the election, and that White House officials had engaged in numerous deliberations about how much to disclose before Election Day.

“There was an honest and ethical determination among the senior leadership not to undermine the election,” the official said. “The political sensitivity was extremely important to take into account.”

After failing to get bipartisan backing from senior members of Congress for a joint statement condemning Russian interference, the White House released language from National Intelligence Director James R. Clapper and Homeland Security Secretary Jeh Johnson accusing Russia of interfering in the election.

Last week, President Obama ordered all U.S. spy agencies to cooperate on a fuller investigation and to deliver a consensus report on Russian interference before Trump is inaugurated.

Ellen Nakashima, Greg Jaffe and Julie Tate contributed to this report.