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.

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."

Related: Why Obama Didn't Do More About Russian Hack

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. It was never a literal phone, instead beginning as teletype more than 50 years ago, then converting to first fax and then email to link the U.S. and Russia through their Nuclear Risk Reduction Centers.

In 2013, relying on the pre-existing links with Russia through the Nuclear Risk Reduction Centers, the Obama administration added a channel intended to send email messages and attachments about cyber incidents. This Cyber CBM channel is one of a series of cyber-related confidence-building measures meant to address the need for secure and reliable lines of communication about cybersecurity issues of national concern.

The Obama administration had never used the cyber line before, officials said. Several intelligence officials told NBC News the use of the system 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 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.

Related: FBI Agrees With CIA Assessment That Russia Wanted Trump to Win

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."