As Democrats often do, he paid tribute to Mr. McCain’s military service and his more than five years as a prisoner of war in Vietnam. That is not sufficient qualification for the presidency, Mr. Biden said in as direct a way as any Democrat this year has.

“The choice in this election is clear,” Mr. Biden said. “These times require more than a good soldier. They require a wise leader,” he said, a leader who can deliver “the change that everybody knows we need.”

Part of Mr. Biden’s assignment on Wednesday night was to connect with struggling American families and to portray himself and Mr. Obama as products of the middle class who would fight to restore the American dream. He spoke lyrically of riding the train home to Wilmington, Del., every night and imagining the conversations in the homes he passed.

“Like millions of Americans, they’re asking questions as profound as they are ordinary,” he said. “Questions they never thought they would have to ask.” Whether to bring an aging parent into their homes to save money. How to pay for gas and groceries. Whether to defer a college education. How to pay for retirement.

Mr. Biden, who referred to his childhood struggle with stuttering, made a few verbal slips, including referring to Mr. McCain as “George.”