With his fired-up return to the trail on Tuesday, Mr. Obama showed again why he is Mrs. Clintonís most valuable surrogate. He is the most popular person in national politics and perhaps uniquely effective at reaching black voters and young people at a moment when Democrats are anxious about turnout. That makes him a powerful asset to Mrs. Clinton in a state like North Carolina, where he will campaign on Wednesday.

But for Mr. Trump, who has striven to deliver a strict message over the last week, Mr. Obama is also a uniquely dangerous figure, unusually skilled at goading Mr. Trump into confrontation and anger. Though a fight with Mr. Obama would be politically ill advised, Mr. Trump may struggle to not take the bait if Mr. Obama continues with the kind of onslaught he opened on Tuesday.

Does anyone go positive?

In a campaign fought largely through scorched-earth attacks, the final week might always have been destined to look like this.

Yet the daily crossfire between the campaigns is still striking for its total and ferocious negativity: Mr. Trump has staked his underdog campaign on savaging Mrs. Clinton as corrupt, while Mrs. Clinton and her aides have battered Mr. Trump for his personal finances and the accusations of sexual assault against him, as well as for Mr. Trumpís warm attitude toward Russia.

If Mr. Trump has been a slash-and-burn campaigner from the start, Mrs. Clinton has gestured here and there toward uplift: She said on Tuesday that she hoped to persuade people to vote for her, not merely against Mr. Trump. But even a concerted effort to go positive might be unlikely to break through at this stage.