PHILADELPHIA — Led by President Barack Obama, Democrats sought Wednesday night to cast Hillary Clinton as the lone presidential candidate qualified on economic and security issues and paint Donald Trump as unfit to serve.

Obama emphasized Clinton’s experience — particularly as his secretary of state — over Trump’s.

“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,” Obama said.

“And no matter how daunting the odds; no matter how much people try to knock her down, she never, ever quits. That’s the Hillary I know,” the president said. “That’s the Hillary I’ve come to admire. And that’s why I can say with confidence there has never been a man or a woman more qualified than Hillary Clinton to serve as president of the United States of America.”

At the end of the speech, Clinton joined Obama onstage, embracing him and waving to the crowd.

Political commentators said that Obama, like other speakers in the night’s lineup, was working to appeal to the center of the electorate.

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MSNBC correspondent Joy Reid said the country’s more racially and religiously diverse demographics haven’t upended their fundamental values. “He was arguing right to the middle of the country that ‘this is the America that you know,’ ” she said of Obama.

Steven Schmidt, a GOP campaign strategist on MSNBC’s panel, noted that Obama quoted Republicans Ronald Reagan and Theodore Roosevelt and sounded more hopeful notes than speakers at the GOP convention. Schmidt said the president knows “as a student of American political history that when you sever optimism from . . . the right-of-center party, the right-of-center party never does well.”

On Fox News, former New York City Mayor Rudy Giuliani, a Trump surrogate, criticized the Democratic convention for addressing “no substantive issue” and trying to distract from real issues at stake.

“This president has left this country in the most dangerous state that it’s been in since the Cold War,” Giuliani said.

Obama was the final speaker on a night in which Democrats rolled out a string of big-name speakers — including Vice President Joe Biden and former New York City Mayor Michael Bloomberg — to make the case for Clinton. They talked about issues including foreign policy, college costs and gun violence.

In a 42-minute address, Obama touched on what he believes is his legacy, including health care expansion, an economy revived from the recession and the elimination of Osama bin Laden.

He stressed Clinton’s credentials, while saying of Trump: “Not really a plans guy. Not really a facts guy, either.”

Focusing on the fight against terrorism, Obama said, “Our troops have pounded ISIL without mercy, taking out leaders, taking back territory. I know Hillary won’t relent until ISIL is destroyed.”

Clinton will “finish the job — and she’ll do it without resorting to torture, or banning entire religions from entering our country,” Obama said. “She is fit and she is ready to be the next commander in chief.”

It was not the only theme of the night. The other was questioning Trump’s fitness for office.

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“Does anyone really believe that a guy who’s spent his 70 years on this Earth showing no regard for working people is suddenly going to be your champion? Your voice?” Obama asked. “If so, you should vote for him.”

“But if you’re someone who’s truly concerned about paying your bills, and seeing the economy grow, and creating more opportunity for everybody, then the choice isn’t even close,” said Obama.

“If you want someone with a lifelong track record of fighting for higher wages, better benefits, a fairer tax code, a bigger voice for workers, and stronger regulations on Wall Street, then you should vote for Hillary Clinton,” Obama said.

Talking about the American character, Obama said: “That’s why anyone who threatens our values, whether fascists or communists or jihadists or homegrown demagogues, will always fail in the end.”

Vice President Joe Biden said of Trump: “No candidate of a major party in the United States has ever known less . . . about national security.”

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Earlier, Trump had called on Russia to hack emails from Clinton — a suggestion many Democrats saw as encouraging a foreign adversary to engage in espionage to influence the election. Trump later tried to say his remarks were meant to prod Russian hackers to turn over any emails.

But Democrats saw an opening for a new attack on the Republican.

“As someone who was responsible for protecting our nation from cyberattacks, it is inconceivable to me that any presidential candidate would be this irresponsible,” said Leon Panetta, a former defense secretary and former CIA director.

Bloomberg gave stinging criticism of Trump’s business practices, citing a trail of bankruptcies, lawsuits and contractors who felt cheated.

“Trump says he wants to run the nation like he’s run his business? God help us,” Bloomberg said.

Nearly overshadowed was Sen. Tim Kaine (D-Va.), Clinton’s running mate and a former Virginia governor.

Kaine, 58, who has faced criticism from the party’s left wing on issues including trade, introduced himself as the child of an ironworker with a Jesuit education, who worked as a missionary in Honduras. He sprinkled Spanish throughout his speech, drawing cheers, and noted that he has a son who was just deployed with the U.S. Marines.

Kaine, too, focused on the “trust” theme while invoking his son.

“I trust Hillary Clinton with our son’s life,” Kaine said.

Kaine also sought to turn around the credibility question, saying Trump is the one with the trust problem.

“Does anybody in this massive arena believe there’s nothing suspicious in Donald’s tax returns?” Kaine said, referring to Trump’s refusal to release his returns.

“He just says, ‘Believe Me,’ ” Kaine continued. “Here’s the question: Do you believe him? Donald’s whole career says you better not.”

Biden, warmly received by party regulars and the progressive wing, talked of humble beginnings, family tragedy (his first wife was killed in a car accident and his son died of brain cancer), and cast himself as “Middle Class Joe” — a social status he said Trump doesn’t understand.

“He’s trying to tell me he cares about middle class. Give me a break, that’s a bunch of malarkey,” Biden said to roars. “This guy doesn’t have a clue about the middle class. Not a clue.”

That set off an arena-wide chant of “Not a clue. Not a clue.”

With Emily Ngo