Determined to stop Russia's interference in the presidential campaign, at least one of President Obama's senior advisers urged him to make the ultimate threat to Russian President Vladimir Putin, U.S. officials told NBC News:
Mess with the vote and we will consider it an act of war.
But Obama opted not to issue a warning that specific when he spoke to Putin about the hacking during a September meeting at the G-20 summit in China, said intelligence officials offering NBC News exclusive new details.
The president didn't want to inflame an already tense situation, the officials said. Instead, he used less specific language to warn Putin of consequences if Russian interference didn't stop.
The release of hacked Democratic emails continued.
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A month later, the U.S. used the latest incarnation of an old Cold War communications system — the so-called "Red Phone" that connects Moscow to Washington — to reinforce Obama's September warning that the U.S. would consider any interference on Election Day a grave matter.
This time Obama used the phrase "armed conflict."
"International law, including the law for armed conflict, applies to actions in cyberspace," said part of a message sent over the Red Phone on Oct. 31, according to a senior U.S. official. "We will hold Russia to those standards."
The so-called "Red Phone" system is used to communicate in moments of crisis, such as the September 11, 2001 terror attacks or the U.S. invasion of Iraq. Formally known as the Nuclear Risk Reduction Center line, it is no longer a literal phone, and instead sends email messages and attachments.
The Obama administration had never used the system before, officials said. Several intelligence officials told NBC News the use of the Red Phone communicated just how serious the situation had become.
"It's a dramatic step to pick that phone up and use it," said Ret. Adm. James Stavridis, the former head of NATO.
Did the Red Phone message work? "Look at the results," said an Obama administration official. "There was nothing done on Election Day, so it must have worked."
He meant that the nightmare scenario officials feared didn't occur: An attempt by Russia to manipulate the voting results or throw them into question.
Having lost a close election, however, senior Democrats don't see the White House pushback against Putin as a success. The Russians had already stolen reams of emails and used them to embarrass Hillary Clinton and the Democrats. Whether those revelations swung the election can never be known, but Democrats say they played a major role in an election decided by fewer than 80,000 votes spread across three states.
Donna Brazile, a Democratic party official, has said publicly that attempted cyber intrusions of Democratic Party systems continued through Election Day — and that Democrats felt abandoned by the government.
A senior intelligence official told NBC News the message ultimately sent to the Russians was "muddled" — with no bright line laid down and no clear warning given about the consequences. The Russian response, said the official, was non-committal.
"The Ultimate Red Line"
In a statement, a senior White House official confirmed that the Red Phone was used on Oct. 31 to send a message to Moscow. "This action was part of our ongoing, rigorous efforts to press the Russian government to halt the actions of those responsible for these cyber attacks." The official declined to provide details of the message, however.
Obama claimed success for his message to Putin at a news conference last Friday.
"I felt that the most effective way to ensure that that didn't happen was to talk to him directly and tell him to cut it out and there were going to be serious consequence if he didn't," said Obama. "In fact, we didn't see further tampering of the election process."
Stavridis, whom Hillary Clinton considered as a potential running mate, said he believed the U.S. response to the Russian hacks "was insufficient to achieve the objective of ensuring that we do not receive further attacks. In other words, when someone tries to bully you, you have to push back or you invite further response."
But he also said the word "war" has to be deployed "extremely carefully."
"I think declaring something an act of war, let's face it, is the ultimate red line."
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BEIJING, CHINA - NOVEMBER 10: U.S. President Barack Obama (L) and Russian President Vladimir Putin (R) attend a family photo ceremony during the APEC Leaders meeting November 10, 2014 in Beijing, China. The APEC Summit hosted 1500 economic leaders in Beijing to deliberate key issues facing the Asia-Pacific economy. (Photo by Sasha Mordovets/Getty Images)
SAINT PETERSBURG - SEPTEMBER 05: U.S. President Barack Obama (R) greets Russian President Vladimir Putin at the G20 summit on September 5, 2013 in St. Petersburg, Russia. The G20 summit is expected to be dominated by the issue of military action in Syria while issues surrounding the global economy, including tax avoidance by multinationals, will also be discussed during the two-day summit. (Photo by Sasha Mordovets/Getty Images)
Russias President Vladimir Putin (L) walks past US President Barack Obama as he arrives to pose for the family photo during the G20 summit on September 6, 2013 in Saint Petersburg. World leaders at the G20 summit on Friday failed to bridge their bitter divisions over US plans for military action against the Syrian regime, with Washington signalling that it has given up on securing Russia's support at the UN on the crisis. AFP PHOTO / JEWEL SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
ENNISKILLEN, NORTHERN IRELAND - JUNE 18: Leaders (L-R) Russia's President Vladimir Putin, Britain's Prime Minister David Cameron, US President Barack Obama stand for the 'family' group photograph at the G8 venue of Lough Erne on June 18, 2013 in Enniskillen, Northern Ireland. The two day G8 summit, hosted by UK Prime Minister David Cameron, is being held in Northern Ireland for the first time. Leaders from the G8 nations have gathered to discuss numerous topics with the situation in Syria expected to dominate the talks. (Photo by Matt Cardy/Getty Images)