President Trump meets with Russian Foreign Minister Sergey Lavrov, left, and Russian Ambassador Sergei Kislyak at the White House in May 2017. (Russian Foreign Ministry/AP)

Walter Dellinger was head of the Office of Legal Counsel at the Justice Department from 1993 to 1996. Samantha Goldstein is a Washington attorney.

In the face of an unprecedented attack on American democracy by a hostile foreign power, Donald Trump and those who worked for him both as a candidate and a president failed to defend the United States. For this alone, he is unfit to hold office.

That is the inevitable conclusion from the Mueller report’s account of how Trump and his campaign knew of, benefited from, welcomed and did nothing to stop the Russian assault.

This is not the aspect of the report that is most damaging to the president — that would be his multiple attempts to obstruct justice, efforts that would likely have led to his indictment were it not for the Department of Justice opinion saying sitting presidents should not be indicted.

But Russia is what proves his unfitness to serve. Every president swears to “preserve, protect and defend the Constitution.” And everyone who works for him swears to protect the Constitution “against all enemies foreign and domestic.” The Mueller report demonstrates that the president and his closest aides violated these fundamental duties imposed by their oaths of office.

The Mueller report’s unfolding narrative of the Russian attack and Trump’s surrender in the face of it is devastating for someone who was offering himself as a future commander in chief.

Words can hardly describe the audacity of the Russian military’s efforts to disrupt the 2016 presidential election. Some have suggested that this attack on our country was somehow made less serious because no Trump campaign officials were indicted for playing a role in the Russian efforts.

The absence of criminal indictments on the matter of cooperation with the Russians, however, does not diminish the gravity and shamefulness of Trump’s failure to confront this military assault. First of all, it is a mistake to assume that conduct that is not a violation of the federal criminal code is less reprehensible than conduct that is criminal. There is no provision of U.S. law, for example, that makes it a federal crime for a president to refuse to defend the United States against foreign attack. But such a refusal surely is deplorable.

When it comes to violations of the criminal code, Mueller set a high bar for indicting any Americans for “collusion” with the Russian attack. To indict Americans, Mueller states, it would be necessary to be able to prove beyond a reasonable doubt that those people coordinated with the Russians, with “coordination” in turn requiring an actual agreement (tacit or explicit) between campaign associates and Russian operatives.

This high bar is appropriate; we should think carefully before deploying the criminal law against political candidates and their campaign officials. But what Mueller finds in his report is activity by the Trump campaign that was incompatible with true allegiance to the United States and its democratic system of government. Mueller shows that campaign officials, including members of the president’s family, knew Russian military operatives were seeking to influence the outcome of the election, welcomed it, and even encouraged it. What they did not do is bring in the FBI or any other counterintelligence agency. Instead, Trump denigrated those institutions and ignored their unanimous conclusion that the Russians’ concerted assault posed a serious threat to the United States. To this day, Trump has failed to respond to the Russians’ military intervention or to take steps to ward off future attacks.

No one who reads Mueller’s account of the Russian intrusion can seriously doubt that it was imperative for the FBI and other agencies to investigate the Russian assault. The Russians committed serious crimes against the United States as part of a coordinated multiyear attack against U.S. democracy, conducted in part within our boundaries, by a hostile foreign power.

Faced with these treacheries, the Trump campaign did worse than nothing. For instance, while running for president, Trump called on the Russians, saying “Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing.” His eldest son arranged a meeting with a lawyer connected to the Russian government in the hopes of getting “dirt” on that opponent. A Trump campaign foreign policy adviser pursued meetings with Russian government officials in pursuit of damaging information about Hillary Clinton. And during at least one campaign rally, Trump publicly declared “I love WikiLeaks.”

Even after he was elected president, Trump undertook no serious effort to respond to the Russians’ coordinated assault. Instead, he played down the seriousness of their attacks, tweeting that claims about Russian interference were an excuse by Democrats for losing the election and calling U.S. intelligence agencies’ investigation of Russia’s attacks a “WITCH HUNT!” His State Department also failed to spend more than $100 million of congressional appropriations for counter-propaganda work, which could have been used to fight back against Russian disinformation. Trump even declared publicly that he had discussed with Russia’s President Vladimir Putin the possibility of setting up a joint Russian-American cybersecurity unit.

In Mueller’s words, the "Russian government interfered in the 2016 presidential election in sweeping and systematic fashion.” Russia’s success in infiltrating our most recent presidential election makes clear that Russia poses an ongoing threat to the integrity, safety and security of the United States.

During the 2016 election, American democracy was under Russian attack. Candidate Trump encouraged it. When it was over, President Trump did nothing about it.

Read more:

Michael McFaul: Russia attacked us. The Mueller report still doesn’t give us the full story.

David Von Drehle: Donald Trump is lucky. Vladimir Putin is luckier.

Max Boot: To impeach or not to impeach? That is now the question.

George J. Terwilliger III: William Barr gave this country an extraordinary gift

George Conway: Trump is a cancer on the presidency. Congress should remove him.