CHICAGO (AP) — Warning of a difficult fight ahead, former President Barack Obama and Michelle Obama on Tuesday called on the nation to embrace Kamala Harris in urgent messages to the Democratic National Convention that were at times both hopeful and ominous.

"America, hope is making a comeback," the former first lady declared. She then tore into Republican Donald Trump, a sharp shift from the 2016 convention speech in which she told her party, "When they go low, we go high."

"His limited and narrow view of the world made him feel threatened by the existence of two hardworking, highly educated, successful people who also happened to be Black," Michelle Obama said of Trump.

Barack Obama, the first Black president in U.S. history, insisted the nation is ready to elect Harris, who is of Jamaican and Indian heritage and would be the nation's first female president. He also called Trump "a 78-year-old billionaire who hasn't stopped whining about his problems since he rode down his golden escalator nine years ago."

"It's been a constant stream of gripes and grievances that's actually gotten worse now that he's afraid of losing to Kamala," he said.

The fiery messages from two of the Democratic Party's biggest stars underscored the urgency of the moment as Harris works to stitch together a broad coalition in her bid to defeat Trump this fall. The vice president is drawing on stars like the Obamas and other celebrities, officials from the far left to the middle, and even some Republicans to boost her campaign.

And while the theme of the night was "a bold vision for America's future," the disparate factions of Harris' evolving coalition demonstrated, above all, that they are connected by a deep desire to prevent a second Trump presidency.

In an appearance perhaps intended to needle Trump, his former press secretary Stephanie Grisham — now a harsh critic of her former boss — also took the convention stage.

Trump "has no empathy, no morals and no fidelity to the truth," Grisham said. "I love my country more than my party. Kamala Harris tells the truth. She respects the American people. And she has my vote."

Sens. Chuck Schumer, the Senate Democratic leader, and Bernie Sanders, the Vermont independent beloved by progressives, both praised Harris.

Schumer called on voters to elect another Democratic majority to the U.S. Senate. "She can't do it alone," he said of a prospective President Harris.

Sanders, a self-described democratic socialist, said he was eager to work with Harris in the White House as well. Their policy goals, he said, are "not a radical agenda."

As Democrats addressed the nation from Chicago, Harris faced an estimated 15,000 people in battleground Wisconsin in the arena where Republicans held their convention last month. She said that she was running "a people-powered campaign."

"Together we will chart a new way forward," the vice president said in remarks that were partially broadcast to the DNC. "A future for freedom, opportunity, of optimism and faith."

Still, it was not all serious on the second night of the four-day convention.

A symbolic roll call in which delegates from each state pledged their support for the Democratic nominee turned into a party atmosphere. A DJ played a mix of state-specific songs — and Atlanta native Lil Jon ran out during Georgia's turn to his hit song with DJ Snake, "Turn Down for What," to the delight of the thousands inside the cavernous United Center.

And various speakers offered personal stories about Harris, who has served as a California senator and vice president, but remains largely unknown among many voters.

Second gentleman Doug Emhoff, who would become the nation's first gentleman if his wife wins the presidency, shared details about his relationship with the vice president — their cooking habits, their first date and her laugh, which is often mocked by Republican critics.

"You know that laugh. I love that laugh!" Emhoff said as the crowd cheered. Later, he added, "Her empathy is her strength."

Trump, meanwhile, was out on the campaign trail as part of his weeklong swing-state tour during the Democratic convention. He went to Howell, Michigan, on Tuesday and stood aside sheriff's deputies as he labeled Harris the "ringleader" of a "Marxist attack on law enforcement" across the country.

"Kamala Harris will deliver crime, chaos, destruction and death," Trump said in one of many generalizations about an America under Harris.

Throughout their convention, Democrats have sought to balance a message of unity with an embrace of diversity.

Barack Obama's speech Tuesday night made perhaps the most forceful case for that model as a logical step forward for a bitterly divided nation. In contrast to the party's rhetoric in the recent past around race, Obama framed the Democrats' approach as "a new way forward" for a modern society in contrast to a "divisive," "old" and "tired" strategy of vision offered by the party's chief opponent, Trump.

Michelle Obama also addressed race directly as she jabbed Trump, referencing a comment he made in a June debate.

"Who's going to tell him that the job he's currently seeking might just be one of those 'Black jobs'?" she said. "It's his same old con: doubling down on ugly, misogynistic, racist lies as a substitute for real ideas and solutions that will actually make people's lives better."

Barack Obama returned to the convention stage 20 years after making his first appearance at a national convention, a 2004 appearance in Boston that propelled him into the national spotlight ahead of his successful presidential run. And he praised President Joe Biden, who ended his reelection bid last month and endorsed Harris.

"History will remember Joe Biden as a president who defended democracy at a moment of great danger," Obama said Tuesday as the crowd chanted, "Thank you, Joe." "I am proud to call him my president, but even prouder to call him my friend."

Harris, meanwhile, cast the election in dire, almost existential terms. She implored Americans not to get complacent in light of the Supreme Court decision carving out broad presidential immunity, a power she said Trump would abuse.

She has also seized on Trump's opposition to a nationally guaranteed right to abortion.

"They seemingly don't trust women," she said of Trump and his Republican allies. "Well, we trust women."

The vice president's speech in Milwaukee evoked some of the same themes that underlaid Biden's case for reelection before he dropped out, casting Trump as a threat to democracy. Harris argued that Trump threatens the values and freedoms that Americans hold dear.

Trump said he would be a dictator only on his first day in office, a quip he later said was a joke, and has vowed as president to assert more control over federal prosecutions, an area of government that has traditionally been left to the Justice Department.

Someone with that record "should never again have the opportunity to stand behind the seal of the president of the United States," Harris said. "Never again."

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Cooper reported from Phoenix. Associated Press writers Matt Brown, Farnoush Amiri and Will Weissert in Chicago contributed to this report.