"Sound familiar? It didn't work in 1992, because we were on the right side of history. And it won't work in 2008 , because Barack Obama is on the right side of history."
Not everyone was convinced Mr Clinton was ready to deploy his oratorical skills in support of the man who beat his wife to the nomination, the BBC's Kevin Connolly in Denver says.
But he banished any doubt in a speech in which he said simply that Barack Obama was ready to be president, our correspondent says, a sentence Obama strategists must have started to fear they would never hear from him.
In American political parlance Mr Clinton "delivered", and may now find himself playing a higher-profile role in the campaign to come, our correspondent adds.
Mr Biden, chosen last week by Mr Obama to be his running mate, said he was honoured to share the stage with Mr Clinton.
He praised Mr Obama and drew parallels between their lives and values, growing up in families that had little but worked hard.
"Barack Obama and I took very different journeys to this destination, but we share a common story," he said.
He stressed the need for a new approach to help Americans struggling to make ends meet and to change US foreign policy in the rest of the world.
The 65-year-old foreign policy expert was chosen as vice-presidential candidate by 47-year-old Mr Obama partly on account of his experience.
Earlier in the day, Mrs Clinton earlier halted a roll call vote - in which each state, in alphabetical order, declares how many votes were cast for each candidate in the primaries - to call for Mr Obama's nomination by voice vote.
Thursday: Obama to accept nomination with speech in stadium
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