Proving the source of a cyberattack is notoriously difficult. But researchers have concluded that the national committee was breached by two Russian intelligence agencies, which were the same attackers behind previous Russian cyberoperations at the White House, the State Department and the Joint Chiefs of Staff last year. And metadata from the released emails suggests that the documents passed through Russian computers. Though a hacker claimed responsibility for giving the emails to WikiLeaks, the same agencies are the prime suspects. Whether the thefts were ordered by Mr. Putin, or just carried out by apparatchiks who thought they might please him, is anyone’s guess.

On Sunday morning, the issue erupted, as Mrs. Clinton’s campaign manager, Robby Mook, argued on ABC’s “This Week” that the emails were leaked “by the Russians for the purpose of helping Donald Trump” citing “experts” but offering no other evidence. Mr. Mook also suggested that the Russians might have good reason to support Mr. Trump: The Republican nominee indicated in an interview with The New York Times last week that he might not back NATO nations if they came under attack from Russia — unless he was first convinced that the countries had made sufficient contributions to the Atlantic alliance.

It was a remarkable moment: Even at the height of the Cold War, it was hard to find a presidential campaign willing to charge that its rival was essentially secretly doing the bidding of a key American adversary. But the accusation is emerging as a theme of Mrs. Clinton’s campaign, as part of an attempt to portray Mr. Trump not only as an isolationist, but also as one who would go soft on confronting Russia as it threatens nations that have shown too much independence from Moscow or, in the case of Lithuania, Latvia and Estonia, joined NATO.