In this way, the program was reminiscent of Hillary Clinton’s convention four years ago, when the party also tried to bring along its left flank but spent even more time seeking to portray Mr. Trump as an outlier far removed from the political mainstream.The specter of a Trump presidency back then, however, was a theoretical proposition. This year, Democrats were able to lay out a more powerful indictment based on Mr. Trump’s tenure in the White House. And had Democrats nominated a more liberal candidate than the consensus-oriented Mr. Biden, they might not have been able lure former Republican office holders.Not that every Democrat was happy that Mr. Kasich, an anti-union Republican, was allowed to speak in prime time at the convention: Some influential labor leaders complained bitterly to Mr. Biden’s senior aides about Mr. Kasich’s appearance, according to Democrats familiar with the conversations.For his part, the president largely ignored the Republicans who spurned him, turning instead to racial demagogy. On a swing through the Midwest on Monday, Mr. Trump accused Democrats of representing left-wing extremism and, returning to the xenophobic themes of his first presidential campaign, argued baselessly that Mr. Biden would “overwhelm Minnesota with refugees from terror hot spots.”Appearing at an airplane hangar in front of Air Force One, the president continued his bald campaign to sow doubts about the integrity of the electoral process — rhetoric that no modern president has dared use.Mrs. Obama, alluding to Mr. Trump’s false claims about voter fraud, cited the challenges to voting access that some Democrats fear will prove inevitable this fall, and pleaded with Americans to do whatever it would take to cast their ballots. “We’ve got to be willing to spend all night in line if we have to,” she said.Recognizing that her remarks would not be as powerful without applause, the organizers displayed a Zoom-style panoramic of Democrats applauding the former first lady.