Historians, foreign policy experts, and veteran diplomats all say that the Ukraine scandal threatening Donald Trump's presidency is unprecedented.

President Donald Trump in a July phone call urged Ukrainian President Volodymyr Zelensky, who'd recently been elected, to investigate former Vice President Joe Biden, according to both a memo released by the White House and a whistleblower complaint that focuses heavily on the conversation.

One presidential historian told Insider that what Trump did with Ukraine was not only "profoundly damaging" but also "profoundly stupid."

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"Unprecedented."

It's a word that's been tossed around repeatedly — and often quite lazily — by political pundits during Donald Trump's presidency, but historians and veteran diplomats say it applies to the Ukraine scandal perhaps more so than any other controversy the president has faced in his tenure thus far.

One historian told Insider that what Trump did with Ukraine was not only "profoundly damaging" but also "profoundly stupid."

In a phone call on July 25, Trump urged Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden and his son, Hunter Biden, according to a whistleblower complaint and a phone call summary released by the White House. The elder Biden is a leading contender for the 2020 Democratic presidential nomination and an unabashed Trump critic.

Read more: 'This was his show': Rudy Giuliani emerges as the key to unraveling the Trump-Ukraine whistleblower scandal

Trump has made baseless allegations against Biden and his son over the ousting of a Ukrainian prosecutor who at one point oversaw investigations into a gas company where Hunter sat on the board. There's no evidence of wrongdoing on the part of either Biden, but the president has continued to push this narrative as he maintains that he's done nothing wrong.

The July phone call between Trump and Zelensky is central to the whistleblower complaint from a member of the intelligence community that accuses Trump of "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."

"This interference includes, among other things, pressuring a foreign country to investigate one of the President's main domestic political rivals," added the complaint, which was publicly released on Thursday.

Trump and his allies have dismissed the significance of the complaint because it relies on secondhand information, but acting Director of National Intelligence Joseph Maguire on Thursday told the House Intelligence Committee that the complaint is "in alignment" with a memo on the call the White House released the day before.

Prior to the document's release on Thursday morning, details on the whistleblower complaint trickled out via leaks and a series of reports in the media. The gravity of the allegations against Trump have led House Speaker Nancy Pelosi to launch a formal impeachment inquiry into Trump — and she now has the backing of a majority of House members. Meanwhile, polling suggests public support for impeachment is on the rise.

In short, the Ukraine scandal poses a serious threat to Trump's presidency, and from a broader standpoint places the country in uncharted territory.

'What Trump did with Ukraine violates the Constitution'

Aaron David Miller, a former State Department advisor who is now a senior fellow at the Carnegie Endowment for International Peace, told Insider that though presidents play politics with foreign leaders all the time "what Trump did goes beyond anything that I personally experienced or even was aware of in government."

Miller, who served as an advisor to multiple secretaries of state across both Republican and Democratic administrations, said he cannot think of a situation in his experience that's "analogous" to the current scandal involving Trump and Ukraine.

"What Trump did with Ukraine violates the Constitution," Miller said, and that asking a foreign leader to investigate a political rival "absolutely" amounts to an abuse of power.

Noah Bookbinder, the executive director of the Washington-based advocacy group Citizens for Responsibility and Ethics (CREW), in an op-ed for The New York Times on Wednesday said the White House memo on the July phone call is "far worse than we could have imagined."

Bookbinder said the memo demonstrates that Trump "sought multiple favors from a foreign leader for his personal political gain and in doing so violated his solemn oath to preserve, protect and defend the Constitution."

Bradley Simpson, an expert on US foreign policy and history professor at the University of Connecticut, in a series of tweets said the requests Trump made in the call were "bonkers" and a major departure from what occurs in most conversations between a sitting US president and a recently elected foreign leader (Zelensky has only been president of Ukraine since May).

"What is not normal is for a President to veer radically and inexplicably away from a policy process built up over months and years by a vast foreign policy and national security bureaucracy and ask for things that no one has ever asked for. That is just crazy — or criminal," Simpson said.

'It undermines our democracy and makes a mockery of the presidency'

A presidential historian is also struggling to point to any moments in US history that closely compare to the Trump-Ukraine scandal.

Jeffrey A. Engel, director of the Southern Methodist University Center for Presidential History, on Friday told Insider, "I can think of no other case in American history that rivals what the president has been accused of."

"It's unprecedented," Engel said. "First of all, and I don't think actually this has been discussed enough in the last 96 hours, what Donald Trump did was incredibly stupid ... It undermines our democracy and makes a mockery of the presidency."

"They teach us in graduate school not to use the word 'unprecedented,' because that means you're a lousy historian, but every now and then that case arises," Engel added.

Trump's phone call with Zelensky was one day after former special counsel Robert Mueller testified before Congress on his investigation into Russian election interference, which among other things looked into whether Trump's campaign coordinated with the Kremlin in these efforts.

Engel said that given the gravity of that probe and the timing of the phone call "you'd think the one thing [Trump] would not do is to turn around and the next day ... collude with a foreign power."

"When we talk about how profoundly damaging this is, we also have to take into account how profoundly stupid this is," Engel added. "I think equally important, it is simply just morally wrong to engage a foreign power in this way. But more important than it being morally wrong, who in America doesn't know that this is illegal at this point?"

Read more: Read the full declassified whistleblower complaint about a phone call between Trump and Ukraine's president

Engel said the only other comparable situation that comes to his mind is Richard Nixon's efforts in trying to scuttle Vietnam peace talks as he ran for president in 1968. At the time, Nixon was worried that if too much progress was made he would lose and told an aide to find a way to covertly "monkey wrench" the talks.

But Engel added that there are "thousand differences between these two cases" and expressed shock Trump would dare make such a request of a foreign leader on a White House phone while others were listening in.

The complaint said the call took place in the White House Situation Room and approximately a dozen officials listened to it. "The officials I spoke with told me that participation in the call had not been restricted in advance because everyone expected it would be a 'routine' call with a foreign leader," the whistleblower wrote.

"What Trump has done is essentially destroyed the one — no pun intended — trump card we always had, which is that we were the leaders of the free world. That can longer be said ... That has never been the case up until Trump since 1945," Engel said. "Who in Europe would not laugh if you said, 'Donald Trump is the leader of the free world?'"