Clinton pushed back against attacks - initiated by himself and his wife during the bitter primary campaign, and later taken up by Republican John McCain - that Obama is ill-prepared for the White House, especially on matters of national defense.
But he also suggested that on such weighty issues, Obama would be leaning on his seasoned vice president, six-term Sen. Joe Biden.
"With Joe Biden's experience and wisdom, supporting Barack Obama's proven understanding, insight and good instincts, America will have the national security leadership we need," Clinton said.
Clinton campaigned feverishly for his wife, Hillary Rodham Clinton, in her long-fought primary battle against Obama, and took her loss hard. He had not spoken out strongly in support of Obama since he clinched the nomination in June.
But Wednesday, he was unambiguous in passing the torch of Democratic leadership to Obama.
Jabbing a finger at thousands of cheering delegates, he declared: "I want all of you who supported her to vote for Barack Obama in November."
Running just over 20 minutes, Clinton's speech whipped thousands of delegates into a frenzy. Where a night before they had hoisted "Hillary" banners, on this night they waved American flags, symbols of the unity the fractious party seeks.
The delegates stood on their feet and roared for nearly 3½ minutes when Clinton walked on stage. The former president basked in their affection, but after several false starts at his speech, commanded: "Sit down!"
Clinton was by turns funny, nostalgic and wonkish, touching on issues like health care and pension benefits.
Clinton, ever mindful of himself, likened Obama's presidential quest to his own bid for the presidency in 1992, when "Republicans said I was too young and too inexperienced to be commander in chief."
"Sound familiar?" Clinton said. "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."
He allowed that the primary campaign had generated "so much heat it increased global warming."
"In the end," he said, "my candidate didn't win. But I'm proud of the campaign she ran: She never quit on the people she stood up for, on the changes she pushed for, on the future she wants for all our children."
In an unusual gesture for a partisan address, Clinton offered measured praise for Republican , saying he had served heroically in Vietnam. "He loves our country every bit as much as we all do," Clinton said. "And as a senator, he has shown his independence of right-wing orthodoxy on several issues."
But, he said, McCain "still embraces the extreme philosophy which has defined his party for more than 25 years."
Clinton's address elicited some nostalgia of its own among the delegates.
"He can still mesmerize a crowd," said Oregon delegate Sam Sappington.
Standing nearby, Lloyd Henion, who came to Denver as a Clinton delegate from Oregon, said the former president "hit a 500-foot home run. He hit all the points."
Clinton's challenge Wednesday night was tall, because he himself had questioned Obama's credentials.
During the primary race, the former president tried to raise doubts about whether the first-term Illinois senator had the experience to lead the country. He said Obama's opposition to the Iraq war was a "fairy tale."
Last fall, he dismissed Obama as totally unqualified.
"I mean, when is the last time we elected a president based on one year of service in the Senate before he started running?" Clinton said on "The Charlie Rose Show. "In theory, we could find someone who is a gifted television commentator and let them run."
Last winter, Clinton said that after "all the mean things" the Obama campaign had said about him, "I should be the last person to defend him. (But) if he wins this nomination, I'm going to do what I can to help him win."
Clinton was departing Denver on Thursday morning, hours before Obama gives his acceptance speech.
Aides said this was standard practice for Clinton, and not a snub. Clinton did likewise at the 2000 and 2004 conventions, they said.