John Bacon, and Richard Wolf

USA TODAY

PHILADELPHIA — Democrats stepped up their attacks on Donald Trump Wednesday as they nominated Virginia Sen. Tim Kaine for vice president and heard President Obama call Hillary Clinton the most qualified presidential candidate in history.

Despite a star-studded lineup of speakers that included Vice President Biden and former New York City mayor Michael Bloomberg, it was Trump who increasingly dominated attention at the Democratic National Convention -- in part because of his latest controversial comments urging Russia to unearth Clinton's missing emails.

"What we heard in Cleveland last week wasn’t particularly Republican – and it sure wasn’t conservative," Obama said of the Republican National Convention. "What we heard was a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world. There were no serious solutions to pressing problems – just the fanning of resentment, and blame, and anger, and hate. That is not the America I know."

By contrast, Obama said, "We don’t fear the future. We shape it, we embrace it, as one people, stronger together than we are on our own. That’s what Hillary Clinton understands – this fighter, this stateswoman, this mother and grandmother, this public servant, this patriot – that’s the America she’s fighting for."

"There has never been a man or a woman -- not me, not Bill (Clinton), nobody -- more qualified than Hillary Clinton to serve as president of the United States of America," he said.

Clinton took the stage at the end of Obama's speech, and the two -- the popular president and the presidential nominee who would replace him -- hugged and waved to the cheering delegates.

Earlier, Biden pronounced Trump less prepared for the Oval Office and the nuclear codes than any major-party candidate in history. "He has no clue about what makes America great," the vice president said. Then he ad-libbed: "Actually, he has no clue, period." The arena erupted into a chant, "Not a clue!"

"We cannot elect a man who belittles our closest allies while embracing dictators like Vladimir Putin," Biden said.

In a plain-folks manner that appeased some of Bernie Sanders' supporters -- who continued to disrupt portions of the convention for a third consecutive night in defense of their left-wing agenda -- Kaine mimicked Trump as "a slick-talking, empty-promising, self-promoting, one-man wrecking crew."

"He never tells you how he's going to do any of the things he says he's going to do," Kaine said. "He just says, 'believe me.' So here's the question: Do you really believe him? Donald Trump's whole career says you better not."

Former defense secretary Leon Panetta lauded Clinton's participation in the operation that killed Osama bin Laden and lashed out at Trump's latest invitation for Russian intervention in the presidential campaign.

"It is inconceivable to me that any presidential candidate would be that irresponsible," Panetta said. "In an unstable world, we cannot afford unstable leadership."

The third day of the Democrats' conclave also featured a guest appearance by Bloomberg, a political independent who denounced Trump's record as a Manhattan-based developer and urged delegates to vote for the "sane, competent person" in the race.

"Throughout his career, Trump has left behind a well-documented record of bankruptcies, and thousands of lawsuits, and angry shareholders and contractors who feel cheated, and disillusioned customers who feel they've been ripped off," Bloomberg said.

"The bottom line is: Trump is a risky, reckless and radical choice. And we can't afford to make that choice."

By far the biggest speech of the night came from Obama, 12 years to the day after he electrified the 2004 Democratic convention in Boston with a keynote address that propelled him toward the White House four years later.

With an eye toward supporters of Bernie Sanders who have not warmed to Clinton during the long primary campaign or the sometimes divided convention, the president said: "If you’re serious about our democracy, you can’t afford to stay home just because she might not align with you on every issue. You’ve got to get in the arena with her."

His appearance, coming the night after former president Bill Clinton lauded his wife's career in public service, overshadowed the day's other major event: Kaine's official nomination as the party's candidate for vice president. Given Obama's popularity among the party faithful, Kaine was relegated to speak before him.

Watch live: Democratic National Convention Day 3

Democratic convention: What's happening now

Trump was on the campaign trail Wednesday, telling reporters in Doral, Fla., that Obama is "the most ignorant president in our history" and adding that Clinton "would be even worse."

After the event, Trump made stops in Pennsylvania and was headed for Ohio, energized by a Los Angeles Times poll that showed him leading Clinton, 47%-40%.

Donna Brazile, who took over as interim Democratic Party chairwoman after Debbie Wasserman Schultz stepped down over allegations that her staff favored Clinton over Bernie Sanders, told USA TODAY she was not concerned by current polling numbers.

"Come Labor Day, you'll see Hillary Clinton pulling away again," Brazile said. "There is a path to the White House for the Democratic Party."

Obama: 'There has never been a man or a woman more qualified' than Clinton

There was an air of confidence in the arena Wednesday as speakers and videos portrayed Clinton as a sound leader and Trump as uninformed and erratic.

"This very morning, he personally invited Russia to hack us," retired Navy Admiral John Hutson said. "That's not law and order. That's criminal intent."

Hutson also noted Trump's attacks on Sen. John McCain for getting captured in Vietnam, saying: "Donald, you're not fit to polish John McCain's shoes."

Unlike the Republican convention in Cleveland last week, most speakers in Philadelphia have addressed domestic policy issues. On Wednesday night, that included a lengthy diatribe against the gun lobby, led by survivors of horrific firearms attacks in Orlando, Fla., Charleston, S.C., and Newtown, Conn.

Delegates stood and applauded many of them, but none more than former Arizona congresswoman Gabrielle Giffords, still recovering from being shot in the head in Tucson more than five years ago. She appeared with her husband, retired astronaut Mark Kelly.

“Speaking is difficult for me," Giffords said. "But come January, I want to say these two words: 'Madame President.'"

Hillary Clinton's brand survives a generational shift

Contributing: Eliza Collins, Nicole Gaudiano. Kevin Johnson, David Jackson