It’s rare for historians to identify any act as a first. Donald Trump has given them the ability to do so. PHOTOGRAPH BY PHILIP MONTGOMERY FOR THE NEW YORKER

Three U.S. senators huddled worriedly last night amid the crush of Democrats in the corridors of the Wells Fargo Center, the site of their national Convention. Leaning in closely, in hopes that no one would overhear, one urgently complained to the others that Hillary Clinton’s campaign wasn’t doing enough to counter the bizarre statements made that morning by Donald Trump.

In one of his impromptu press conferences, Trump had all but begged Russia, which U.S. intelligence officials alleged had hacked into the Democratic National Committee’s e-mails, to go back for more. “Russia, if you’re listening, I hope you’re able to find the thirty thousand e-mails that are missing,” the Republican nominee had declared. “I think you will probably be rewarded mightily by our press.”

Trump, now the official candidate of the Republican Party, seemed to be urging Russia, one of America’s most formidable and historically hostile rivals, to engage in cyber espionage against the Democratic nominee. “Someone needs to call for an investigation!” one of the senators in the corridor exclaimed. “It’s a violation of the Logan Act!” he said, referring to the law that criminalizes private citizens interfering with U.S. policy.

Realizing that they had been overheard, the senators insisted that their heated, private strategizing was off the record, and dashed farther down the windowless channel of packed-in humanity that surrounded the arena’s floor. But the frustration and confusion they exchanged caught, in an instant, the tenor and talk of the Democratic campaign. Trump had once again disrupted American politics at the highest level, and even the most practiced and powerful professionals were beside themselves at the inability of anyone on their side to adequately react.

Like a maddeningly provocative amateur sportsman, Trump had once again served a ball that was so wild it was hard to tell if it was a mistake or a deliberate attempt to destabilize all of the accepted rules of the game. In the past, Presidential nominees have taken brief vacations during the opposing party’s Convention, deferentially ceding the spotlight. But Trump, who made in-person appearances nearly every night of his own Convention, had grabbed the world’s attention during the Democratic Convention, too. His statement had been so extraordinary that no one knew whether to dismiss it as a joke (which he later insisted it was) or to take it deadly seriously—as one ordinarily would the words of someone with a near-even chance of becoming the most powerful elected official in the world. Trump and his campaign officials have laughed off suggestions that they had any connections to the murky Russian actors behind the hacking of the D.N.C.’s e-mails, but they also failed to condemn the illegal breach.

Working yesterday morning on the other side of town, at the National Constitution Center, the historian Sean Wilentz was among those who regarded Trump’s conduct as borderline treason. Wilentz, a professor of American history at Princeton (and a longtime supporter of Hillary Clinton), described himself as “flabbergasted.” “I can’t think of anything that has happened like this before,” he said.

It’s rare for historians to identify any act as a first. Ordinarily they are full of caveats, precedents, and nuances. But in this case, Wilentz said, he could think of no comparison in American history. If a sitting President, rather than a candidate, had called for a foreign power to commit an act of espionage in order to influence the outcome of an election, “it would be an impeachable act,” Wilentz said. “Watergate was nothing in comparison with this. This is beyond partisanship. It’s stunning, in my view,” he added.

If Trump’s open plea to Russia shocked Wilentz, he did note that there has been at least one instance of a Presidential candidate clandestinely trying to collude with foreign powers to advantage his campaign. In the fall of 1968, Richard Nixon secretly used an emissary to scuttle peace talks aimed at ending the Vietnam War. At the time, Nixon, the Republican nominee, was locked in a fierce campaign against Hubert Humphrey, the sitting Vice-President to Lyndon B. Johnson, who had declined to run for reëlection. The country was riven by conflict over the unpopular and failing Vietnam War. After dithering for much of the campaign season, Humphrey finally distanced himself from Johnson and promised to unilaterally end the war if elected. His poll numbers began to soar, to Nixon’s dismay. Peace talks in Paris, which had seemed interminably bogged down, began to take on new vigor. An end to the long and bloody war finally seemed in sight. But, instead, unknown to American voters at the time, Nixon opened a private channel to the South Vietnamese officials, convincing them to hold off on accepting peace by promising that he would get them a better agreement if he became President.

Johnson learned of this treachery through eavesdropping conducted by U.S. intelligence agencies. But he was unwilling to speak out against Nixon’s actions for fear of compromising the country’s intelligence capabilities. Instead, he stayed silent as Nixon was elected.

“One difference, though, was that that was done through subterfuge,” Wilentz noted. “If the reports on Trump are accurate, he is openly calling on a foreign government to commit a crime, to affect a Presidential election. As far as I know, we have never seen anything like this before.”

The historian Beverly Gage, who teaches twentieth-century American history at Yale, also regards Trump’s behavior as unique. She noted that there have been other incidents of politicians accusing opponents of treasonous foreign alliances, but they relied on dark conspiratorial theories, not the candidate’s public statements at a press conference.

In the nineteen-fifties, she said, Senator Joseph McCarthy, a Republican from Wisconsin, claimed that both President Franklin D. Roosevelt and President Harry Truman had been involved in collusion with the Communist Party and the Soviet Union for two decades. However, McCarthy wasn’t himself a Presidential candidate. During the 1964 election, the Republican nominee, Barry Goldwater, embraced the John Birch Society, a far-right fringe group whose founder, Robert Welch, had accused President Dwight D. Eisenhower, a Republican, of being a "dedicated, conscious agent" of the Soviet government.

“Of course, in both of these cases, this was a far cry from encouraging a foreign government to spy on the United States,” Gage acknowledged. “For that, despite all of the scandals and mudslinging of past U.S. elections, I cannot think of any clear precedent.”

By picking Philadelphia as the setting for their national political Convention, the Democrats were inviting historic comparisons. They probably hoped that the history they would make was their nomination of the first woman who could conceivably win the Presidency. But it was another historic first that dominated much of the talk during the third day of the Democratic National Convention. As has been a pattern during this strange election year, long-anticipated highlights were again upstaged by historical new lows, and what remained to be seen was how much the country grasped the enormity of the disruption under way, and how much it cared.