"He passed the Affordable Care Act, a goal strived for by presidents since Teddy Roosevelt," he said to cheers. "It required him early on to use up almost all of his political capital. He prevailed where no president had done before."
The same audience booed Romney on Wednesday when he said he would repeal the health-care law.
Obama captured 95 percent of the African American vote in 2008 and is expected to win it again by a resounding margin in November. But with contests expected to be close in swing states such as Florida, Ohio, Pennsylvania, North Carolina, and Virginia, Biden implored his audience to vote in similar - if not greater - numbers this fall.
He called Republicans obstructionists who do not quit.
"Their discipline was amazing," he said. "They never let up. But neither has my guy, neither has Barack Obama. He hasn't given up."
Romney's campaign dismissed Biden's speech.
"The black American community has struggled with high unemployment under President Obama, and Vice President Biden's speech today offered no new ideas or solutions," said Romney adviser Tara Wall.
"Mitt Romney will enact policies that will lower taxes, encourage small-business hiring, reform our education system, and give all Americans an opportunity to pursue their dreams," she said.
Biden urged conventioneers to imagine a Romney Justice Department or Supreme Court.
"Remember what this at its core was all about, what this organization at its core was all about," he said. "It was about the franchise. It was about the right to vote. Because when you have the right to vote, you have the right to change things."
He said that Republican moves to require photo identification at polling places, as well as other new voting access laws in more than a dozen states, were obstacles to the ballot box.