“I need a Republican Senate and a House to accomplish all of the change we have to make,” Mr. Trump said. He added, “I understand and embrace the wisdom of Ronald Reagan’s big tent within the party.”

“Big, big tent,” he said. “Remember?”

He paused to ask the Democrats in the crowd to raise their hands. Hardly anyone made a sound. Mr. Trump, looking unimpressed, offered that he did not need Democrats anyway.