President Obama offered a muscular endorsement of Hillary Clinton and an impassioned defense of his own stewardship of the country Wednesday, capping a roster of political luminaries who rallied around the Democratic presidential nominee and savaged her opponent as unfit - if not dangerous.

Obama, hoping to keep a Democrat in the White House, secure his own legacy, and ensure that his successor also breaks a historic barrier, lent his voice to his onetime rival, telling the Democratic National Convention crowd "there has never been a man or a woman more qualified" to be president than Clinton.

And he tore at Republican Donald Trump's vision of an America in decline.

"America is already great. America is already strong," the president said, a shot at Trump's signature slogan. "And I promise you, our strength, our greatness, does not depend on Donald Trump."

On a night meant to reflect a party united to boost its nominee before a bruising campaign, Obama's remarks followed similar words of praise - and warning - from Vice President Biden and former New York Mayor Michael Bloomberg.

Each had once considered a run against Clinton. Instead, like Sen. Bernie Sanders two nights before, they became her advocates, joining the vice presidential nominee, Virginia Sen. Tim Kaine, and a parade of speakers inside the Wells Fargo Center who focused fully on the looming fall campaign.

For Obama, his speech was a bookend to the 2004 convention speech that launched his rise and something of a valedictory for his presidency. While he said the country still faced challenges - from a changing economy to terrorism and racial tension - the president argued that it has thrived on a spirit of generosity, not fear.

"America has changed over the years. But these values my grandparents taught me - they haven't gone anywhere. They're as strong as ever," he said - as Democrats adopted the tones of American exceptionalism that Republicans have long touted.

He signed off by thanking the crowd "for this incredible journey" and added, in a seeming nod toward Clinton, "Let's keep it going."

The crowd roared, she appeared on stage, and the two shared a warm hug.

But much of the convention's third day provided more pointed messages - some of the sharpest attacks yet on Trump - aimed likely at the millions of viewers and undecided voters.

The most notable may have come from Biden, who strode onto the stage to the "Rocky" theme song, and pummeled the GOP presidential nominee's qualifications, platform, and often divisive message.

"This guy doesn't have a clue about the middle class - not a clue," Biden said, sparking cheers.

It was exactly the kind of broadside the Clinton team plans to use to undercut Trump's strength with blue-collar voters in swing states such as Pennsylvania and Ohio.

Obama has also been open in his disdain of Trump.

He had signaled for some time his eagerness to join the fray this campaign, hoping to aid Clinton, the former senator and secretary of state aiming to become the first woman president. As he took the stage, thousands of delegates hoisted signs with his name, as so many did eight years earlier.

Comparing the nominees, he cited Clinton's experience in his cabinet, and her firsthand view of national security decisions, as one of her most important advantages over the New York billionaire.

"Nothing truly prepares you for the demands of the Oval Office," he said. ". . . Until you've sat at that desk, you don't know what it's like to manage a global crisis, or send young people to war. But Hillary's been in the room - she's been part of those decisions."

His was the last in a barrage of attacks on Trump's fitness to make decisions about war and peace. Former military and national security officials lashed the Republican for his incendiary statements and inexperience.

The speeches came hours after Trump, renewing his own attacks on Clinton, told thousands of supporters at a rally in Scranton that she would be "a disaster" for the country and he promised to bring back jobs.

"Our country does not feel 'great already' to the millions of wonderful people living in poverty, violence and despair," he later wrote on Twitter.

Biden, in what might have been the last national speech of his long political career, touted his own earthy roots in Scranton and called Trump's appeal to the middle class "a bunch of malarkey."

He said Trump showed a lack of empathy by making famous the phrase "You're fired."

And he slammed Trump for his inexperience in international affairs. "No major-party nominee in the history of this nation has ever known less or has been less prepared to deal with our national security," Biden said.

Retired Rear Adm. John Hutson thrilled the Democratic audience when he chastised Trump for denigrating Republican Sen. John McCain's time as a prisoner of war.

"Donald," Hutson said, "you're not fit to polish John McCain's boots."

Leon Panetta, the former CIA director, said, "We cannot afford an erratic finger on our nuclear weapons."Bloomberg, a billionaire businessman and onetime Republican, derided Trump's business affairs and his stability.

"I'm a New Yorker, and I know a con when I see one," Bloomberg said. He endorsed Clinton and called on voters to "elect a sane, competent person."

Despite an outburst of "no more war!" chants during a speech by Panetta, most of the protests that had marked Monday and Tuesday eased for much of Wednesday night.

Inside the Wells Fargo Center, Kira Willig, a Sanders delegate from Miami, held a sign reading: "Bernie has my heart but Hillary has my vote, Dems United!"

Babs Siperstein, a Clinton superdelegate from New Jersey and the first transgender member of the Democratic National Committee, smiled at the thought of seeing Obama pass the Democratic mantle to Clinton. She thought back to the Facebook video she had watched Wednesday morning of her youngest grandson's first steps.

"I'm going to have a great story to tell him when he gets older," she said.


Staff writer Tom Fitzgerald contributed to this article.