Former CIA Director John Brennan described US President Donald Trump's failure to acknowledge Russia's election meddling in a joint press conference with Russian President Vladimir Putin on Monday as "treasonous."

Two experts in constitutional law talked to Business Insider about what exactly treason means per the law and whether Trump could be guilty of it.

While they slightly disagreed on the chances of Trump being impeached, they both said his conduct at the Helsinki summit could have other legal implications later on.

After a joint press conference on Monday in which US President Donald Trump, standing next to Russian President Vladimir Putin, refused to endorse the US intelligence community's consensus that Russia interfered in the US 2016 election, former CIA Director John Brennan accused Trump of treason.

"Donald Trump's press conference performance in Helsinki rises to & exceeds the threshold of 'high crimes & misdemeanors,'" Brennan tweeted. "It was nothing short of treasonous."

#Traitor became a trending hashtag on Twitter, with thousands of people expressing outrage at Trump's comments and also accusing Trump of treason.

Here's what exactly treason means and the potential legal consequences of Trump's comments, according to two experts in constitutional law.

The legal definition of treason

Treason is the only crime explicitly defined in the US Constitution, in Article III, Section 3.

It says: "Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court."

This definition of treason is narrow and requires a high burden of proof. Someone can commit treason against the US in favor of another nation only if the US is at war with that country. It also requires at least two witnesses or the traitor's confession.

For Trump to have committed treason, it must first be proved that the US is at war with Russia. But the terms "aid and comfort" are deliberately ambiguous, giving courts plenty of room to interpret what those conditions mean.

What the experts say

Andrew Wright, an assistant White House counsel to President Barack Obama who's now an associate professor at Savannah Law School, told Business Insider on Monday that he didn't believe the US and Russia are at war or that Trump's conduct at the summit alone amounted to treason.

"It's quite clear he's selling out important American national-security interests by not standing up to Russian aggression," Wright said. "That's why you see some people using the term 'traitor.' It's not a term I prefer to use ... It's the kind of thing I'd like to see after more investigative processes and legal findings."

Jens David Ohlin, a vice dean and professor of law at Cornell Law School, told Business Insider that even without a formal declaration, there is a case to be made that Russia and the US are indeed at war.

"One argument would be that Russia has engaged in a covert cyber intervention against US interests, including election meddling, that rises to the level of hostilities," he said.

"However, an even better argument would be that Russia and the United States are on the opposite sides of various armed confrontations in Syria," he continued, referring to Russia's backing of the Syrian government while the US backs rebel groups there.

What's next for Trump

Wright and Ohlin both said Trump's comments opened him up to greater legal liability, but to differing degrees.

While Ohlin said the president's conduct at the summit increased the chances of his impeachment, Wright argued that they would increase only if Trump were proved to have been blackmailed in some way by Russia.

"Trump is clearly helping Russia — whether it rises to the level of 'aid and comfort' would be for a jury to decide or for the House of Representatives to decide if it pursues articles of impeachment," Ohlin said.

Ohlin added that while most would expect a Democratic House to impeach Trump on charges of obstruction of justice related to his attacks on the FBI and special counsel's Russia investigation, Trump's latest defense of Russia at the expense of the US made it "more likely" that the House, should Democrats win the majority in this year's midterm elections, would pursue impeachment on charges of "treason or some other disloyalty-based allegation."

Even if Congress doesn't impeach Trump, his siding with Russia over his own intelligence agencies will certainly be scrutinized by the special counsel Robert Mueller's team, Ohlin said.

"It looks like the 'quo' of a 'quid pro quo' — Putin helped Trump get elected, and now Trump is rewarding Putin with a favorable foreign policy," he said. "If Trump gets listed as an unindicted co-conspirator, this arrangement might be relevant."

Wright was slightly more skeptical about the scope of the legal implications of Trump's comments.

"I don't think the president's conduct of foreign policy — even if I think it's misconduct of foreign policy — is going to itself have legal liability," he said, adding that it would "all really depend on what the findings" of the Russia investigation are.

Wright argued that Trump's behavior in the press conference would be unlikely to lead to criminal consequences unless it were proved that his defense of Russia was the result of his being blackmailed or extorted in some way by Putin or other Russian operatives.

But Wright also said the level of backlash against Trump's remarks from leaders in his own party would most likely "lead to an uptick in oversight activity" in Congress and could signal that the Republican Party's patience for Trump is wearing thin.

"As one of my colleagues put it: Every political figure has a candle of goodwill that they slowly burn down, and it eventually gets to a point where they can't keep the flame alive," Wright said.