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."