Right now, this race is a referendum on Mr. Trump and his leadership. For months, Joe Biden has kept a lower profile, allowing the president’s inability to stay away from controversy to drive the race.But given Mr. Trump’s low job-approval ratings, the president desperately needs this election to be a choice. He also needs voters not to focus on what polls say a majority believes to be true: that Mr. Trump responded inadequately to a pandemic that still rages across the country, upending American life.So when violent protests break out in cities, which are largely led by Democratic politicians, Mr. Trump throws up his hands. The commander in chief proclaims himself to be powerless, in an effort to highlight what he sees as weakness on the part of his opponents. Even as he occupies the highest office in the land, Mr. Trump still considers himself “an outsider,” blaming a political establishment that he now leads for the nation’s problems.“There is violence and danger in the streets of many Democrat-run cities throughout America. This problem could easily be fixed if they wanted to. Just call,” he said. “We have to wait for the call.”This is not a tightly held strategy. The departing White House counselor, Kellyanne Conway, laid it out in fairly blunt terms on Thursday morning: “The more chaos and anarchy and vandalism and violence reigns, the better it is for the very clear choice on who’s best on public safety and law and order,” she said on “Fox and Friends.”That’s why far more of the Republican convention was devoted to attacking Mr. Biden than laying out what, exactly, Mr. Trump would do if elected for a second term.For months, Republicans toggled between attacking Mr. Biden as weak, corrupt and unfit for the role. In this speech, Mr. Trump seemed to settle on a clear line of attack: Mr. Biden is a secret socialist, lacking “the strength to stand up to wild-eyed Marxists like Bernie Sanders and his fellow radicals.” The fact that Mr. Biden ran against Mr. Sanders as an ideological moderate was left out of the narrative.