By Max Fisher, New York Times

WASHINGTON — After all the ways that this year’s presidential election has made history, Donald Trump found a new line to cross on Wednesday, when he said at a news conference that Russia should hack his opponent, Hillary Clinton.

“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing,” Trump said, in reference to the private email server Clinton used while she was secretary of state. “I think you will probably be rewarded mightily by our press.”

There is simply no precedent for this: A presidential candidate publicly appealing to a foreign adversary to intervene in the election on his behalf.

“This is unprecedented — it is one of those things that seems to be genuinely new in international relations,” said Paul Musgrave, a University of Massachusetts professor who studies American foreign policy.

After a long pause, Musgrave added, “Being shocked into speechlessness is not the sort of thing you’re really used to in the business of foreign policy analysis.”

As part of an investigation into her private server, Clinton handed over 30,000 emails to the State Department. But she deleted a similar number of emails that she said were unrelated to her work at the department.

U.S. presidential elections are high-stakes events. Russia would not be the first foreign power, friendly or hostile, to pursue its preferred outcome. Nor would Trump be the first politician to leverage foreign actors for electoral benefit.

But this is the first time that a presidential candidate has openly asked a foreign power to meddle in the democratic process to his benefit. More than that, Trump seemed to be suggesting that Russia should violate U.S. law on his behalf.

Were Russia to follow Trump’s suggestion, the foreign intervention into U.S. politics would be among the most severe of the past century.

In 1940, as the United States debated whether to enter World War II, British spies disseminated rumors to discredit prominent American isolationists and worked to promote politicians who favored intervention.

When President Jimmy Carter ran for re-election in 1980, he lost in part because he had failed to secure the freedom of 52 American hostages held in Iran. They were released on the day of Ronald Reagan’s inauguration. Iranian negotiators later told journalist Mark Bowden that they had stalled to deliberately hurt Carter, as punishment for his having sheltered the former Iranian shah.

Nations pursue their interests, whether other countries like it or not. Great powers in particular, including the United States, often meddle in foreign elections.

But such operations are conducted in secret because they are hostile acts, meant to subvert the will of the targeted country’s population and the sanctity of its institutions. Trump, in openly inviting such foreign interference, was undercutting one of the most fundamental national interests of a democratic state.

“Nobody ever — and I think I can be confident about this — nobody ever stood up at a podium and said, ‘Bring it on,'” said Jeremy Shapiro, a Brookings Institution scholar of foreign policy, referring to Trump’s invitation for a foreign power to meddle in his own country’s politics.

Though rare, previous U.S. politicians have looked abroad for help with votes at home.

In 1968, as President Lyndon B. Johnson’s administration tried to broker peace talks in Vietnam, a Republican activist encouraged South Vietnamese officials to resist the talks, which they did. The activist, who represented herself as speaking for the Republican presidential candidate, Richard M. Nixon, said Nixon would get South Vietnam a better deal.

According to documents that were later declassified, a South Vietnamese official was recorded as saying that his government had refused to participate in the talks as a way “to help Nixon.”

More recently, in 2012, the Republican presidential challenger, Mitt Romney, cultivated ties with Prime Minister Benjamin Netanyahu of Israel. While Netanyahu did not explicitly endorse Romney, he frequently voiced his acute dissatisfaction with President Barack Obama in comments to the American news media.

Three years later, as Obama tried to strike a nuclear deal with Iran, congressional Republicans invited Netanyahu to condemn the proposed accord in a speech to Congress.

While this arguably violated the norms of foreign policy by circumventing the White House on a matter of foreign relations, and by inviting an ally to intervene against the president in a domestic political dispute, Trump went a large step further in soliciting an adversary, and encouraging it to violate U.S. law on his behalf.

In the hours after Trump’s statement, foreign policy and legal analysts struggled to articulate the scale of his deviation from political norms.

William Inboden, a University of Texas professor who served on President George W. Bush’s National Security Council, told Politico the comments were “an assault on the Constitution” and “tantamount to treason.”

Carl Bildt, the former Swedish prime minister and foreign minister, wrote on Twitter, “I never thought a serious candidate for US President could be a serious threat against the security of the West. But that’s where we are.”

It is doubtful that Russia will alter its espionage practices based on public suggestions from Trump, which some defenders argue was meant as a joke. But Musgrave worried that such language could weaken norms, even if only slightly, against foreign involvement in U.S. politics.

“Trump is legitimating behaviors that nobody ever thought could be legitimated,” Musgrave said, calling the incident “one of those reminders about how fragile norms are.”

Shapiro, the Brookings Institution scholar, sounded physically exhausted by Trump’s comments and suggested they were driven by something more banal than collusion with a foreign power.

“To me what it demonstrates is not that he’s necessarily in cahoots with the Russians, or that he’s intending to commit treason or sway the election by this act, but that he just has no sense of what the norms are,” Shapiro said.

“He has no sense of what an extraordinary statement that was.”

To Learn More:

Foreign Politicians Want Trump to Stop Sending Emails Asking for Campaign Donations (by Laurie Kellman, Associated Press)

Rise of Trump Elicits Shock, Outrage and Panic across Europe (by Dan Bilefsky, New York Times)

British Lawmakers Debate Pros and Cons of Banning Trump from Britain (by Kylie MacLellan, Reuters)

Conservatives Decide Trump Qualifies as a Fascist (by Noel Brinkerhoff, AllGov)