In his campaign for president, Donald Trump has not balked at breaking from longstanding party tradition, nor has he shied away from upending diplomatic protocol.

But the Republican nominee’s sustained embrace of Russian President Vladimir Putin, highlighted this week in a candidates’ forum on NBC, has stunned political leaders of both parties who view Putin as a stark adversary. Trump’s stance could also trigger broader global consequences, veering from established American policy on Russia to an extent that experts suggest is unprecedented in modern presidential politics.

Eric Edelman, a former national security adviser to former Vice President Dick Cheney who does not plan to vote for Trump, described Trump’s most recent remarks as “unseemly” and warned they could “embolden Putin ... to take greater risks, say with the Baltic states.”

“It will also undoubtedly create some concern among our allies (beyond the concerns already created by his comments) about our willingness to live up to our treaty obligations for the common defense,” Edelman added.

Trump has consistently praised Putin as a strong leader, even as evidence has mounted that Russia attempted to influence the U.S. election by hacking the Democratic National Committee and other groups and subsequently making their files public. Earlier this year, Trump drew scorn for suggesting that Russia should release emails deleted from Hillary Clinton’s private server, before insisting his remark was sarcastic.

But Trump’s friendly view of Russia was thrown into the spotlight once again Wednesday during a forum moderated by NBC’s Matt Lauer, during which Trump defended his view that Putin is a better leader than President Obama.

“If (Putin) says great things about me, I'm going to say great things about him,” Trump said. “I've already said, he is really very much of a leader.” Trump also cited Putin’s high approval rating among Russians, a score that is likely artificially inflated.

Compounding the impact of Trump’s remarks, Trump called in Thursday for an interview with Larry King on RT America, a television network funded and overseen by the Russian government, widely regarded as a propaganda tool. When King asked Trump whether he thought Russia could be responsible for hacking the DNC, Trump expressed skepticism.

"I think it's probably unlikely. Maybe the Democrats are putting that out; -- who knows?" Trump said. "If they are doing something, I hope that somebody's going to be able to find out so they can end it. Because that would not be appropriate at all."

Multiple reports have characterized U.S. intelligence as having a high level of certainty that Russian intelligence was responsible for the hacks.

Trump running mate Mike Pence defended the celebrity businessman’s view of Putin, telling CNN on Thursday: “I think it's inarguable that Vladimir Putin has been a stronger leader in his country than Barack Obama has been in this country. And that's going to change the day that Donald Trump becomes president.”

Meanwhile, Trump “didn’t know” his interview with King would air “on Russian TV,” Trump Campaign Manager Kellyanne Conway told “CBS This Morning” on Friday, adding that Trump had agreed to the interview because King is “a personal friend.”

But Trump’s remarks and his subsequent interview nevertheless provoked a backlash, including stern rebukes from his Democratic rival, Hillary Clinton, who has sought to portray Trump as unfit for the presidency.

“This is not just unpatriotic and insulting to the people of our country, as well as to our commander in chief,” Clinton said Thursday. “It is scary.”

And, following Trump’s appearance on RT America later Thursday, Clinton added Friday, “It is beyond one’s imagination to have a candidate for president praising a Russian autocrat like Vladimir Putin.”

As Trump’s comments have provided fodder for Democrats, they have also raised alarm among some Republican national security and foreign policy experts.

“I have my issues with President Obama and have said so for seven years now. But anyone who thinks that Vladimir Putin is a more desirable leader needs to have their damn head examined,” said John Noonan, a Republican national security adviser who most recently worked for Jeb Bush’s campaign and opposes Trump. “After that, they could use a civics lesson.”

Trump’s affable posture toward Putin has extended to his policy as well. This summer, the New York Times asked Trump whether NATO countries could “count on the United States to come to their military aid if they were attacked by Russia -- and count on us fulfilling our obligations.”

“Have they fulfilled their obligations to us?” Trump responded. “If they fulfill their obligations to us, the answer is yes.”

Those comments sent shockwaves, suggesting comfort with a weaker NATO, which could empower Russia to encroach into Eastern European states beyond its borders. But Trump’s allies seemed fine with that hypothetical outcome.

“Estonia is in the suburbs of St. Petersburg. The Russians aren’t going to necessarily come across the border militarily. The Russians are going to do what they did in Ukraine,” Newt Gingrich told CBS at the time. “I’m not sure I would risk a nuclear war over some place which is the suburbs of St. Petersburg. I think we have to think about what does this stuff mean?”

Those comments and others, from Trump and his supporters in the GOP, have left some Republican policy experts shaking their heads.

“That, to me, along with the contortions that many Republicans have engaged in since Wednesday night's commander-in-chief-forum on NBC,” Edelman said, “demonstrates the degradation of traditional Republican conservative internationalism that Trump has brought to the party.”

Trump’s stance toward Russia is even starker in the context of recent presidential politics, where the question has not been whether Russia is a foe, but to what degree.

In 2012, during the third presidential debate, Mitt Romney memorably characterized Russia as “without question, our No. 1 geopolitical foe.”

“They fight for every cause for the world’s worst actors,” Romney said. “The idea that [Obama] has more flexibility in mind for Russia is very, very troubling indeed.”

President Obama scoffed at that urgent frame, saying, “The 1980s are now calling to ask for their foreign policy back.”

Noonan worked for Romney’s campaign at that time as a policy adviser. Now, he watches in disbelief as the party’s nominee sends a dramatically different message.

“Few leaders have caused more suffering and conflict today than Vladimir Putin. It shows just how far down the rabbit hole some Trump fans will go to defend their guy,” said Noonan. “It's a national case of Stockholm syndrome, one that makes decent Americans turn their backs on values and traditions that they've held dear for their entire lives.”