But Biden was effective, and at times powerful, speaking as a witness who watched Obama up close. And because of his reputation for saying what’s on his mind, which has often gotten him into trouble, he has a kind of credibility that doesn’t come automatically to those who are always, always on message.

Biden has a gut understanding of white working class and less affluent middle class voters whom Obama needs because, basically, that’s where Biden comes from. That came across when Biden talked about his dad and the auto industry rescue. The people whose jobs were on the line, Biden said, were the people his dad cared about.

That, in turn, gave bite to Biden’s criticism of Mitt Romney, his business background and his leadership of Bain capital. “The Bain Way may bring your firm the highest profits,” Biden said, “but it’s not the way to lead this country from the highest office.”

He also made the strongest case for why the auto bailout was so important. Had the government not stepped in, “there wouldn’t be an auto industry to save.” Letting GM and Chrysler go under would have been an irreversible decision. And losing the auto industry, as Biden argued, would have done symbolic damage, compounding the losses of a shattered auto industry would have inflicted on the economy and to America’s standing.

Biden took on the burden of describing the decision to kill Osama Bin Laden and could do so with relish and offering a degree of detail that would not have been seemly in Obama’s own speech.

Then he turned to the classic vice presidential candidate role: assailing Romney and the Republicans on the deficit, on taxing the wealthy, on immigration, on equal pay. His critique of the Republicans’ attack on “a culture of dependency” went straight at a core Republican weakness: the party’s suspicion of government help of all kinds, even, as Biden argued, help aimed at allowing the less well-off to go to college, improve their skills, and rise up.

And when he choked up toward the end of his speech as he spoke of “the incredible debt we owe to the families of the 6,473 fallen angels and the 49,746 wounded” in a decade of war, the feeling seemed real.

There was a lot of talk over the last three years that Obama would replace Biden on the ticket, perhaps with Hillary Clinton. I never believed any of it — partly because Obama and Biden seem genuinely to like each other, and partly because Obama, a cool student of politics, has always seen Biden as a political asset. Nothing Biden did tonight will cause Obama to change that judgment.