WILMINGTON, Del. — Former President Barack Obama warned that American democracy could falter if President Donald Trump is reelected, a stunning rebuke of his successor that was echoed by Kamala Harris at the Democratic Convention as she embraced her historic role as the first Black woman on a national political ticket.

Obama, himself a barrier breaker as the nation’s first Black president, pleaded with voters Wednesday night to “embrace your own responsibility as citizens — to make sure that the basic tenets of our democracy endure. Because that’s what is at stake right now. Our democracy.”

Throughout their convention, the Democrats have summoned a collective urgency about the dangers of Trump as president. In 2016, they dismissed and sometimes trivialized him. Now they are casting him as an existential threat to the country. The tone signals anew that the fall campaign between Trump and Joe Biden, already expected to be among the most negative of the past half-century, will be filled with rancor and recrimination.

Yet on the third night of the Democrats’ four-day convention, party leaders also sought to put forward a cohesive vision of their values and policy priorities, highlighting efforts to combat climate change and tighten gun laws. They drew a sharp contrast with Trump, portraying him as cruel in his treatment of immigrants, disinterested in the nation’s climate crisis and in over his head on virtually all of the nation’s most pressing challenges.

Democrats also demonstrated a hope that Biden, a 77-year-old white man, can revive the coalition that helped put Obama into office, with minorities, younger voters and college-educated women blunting Trump’s lock on many white and rural voters.

The evening marked a celebration of the party’s leading women, including remarks from Hillary Clinton, the first woman to become a major ticket presidential nominee; House Speaker Nancy Pelosi and Massachusetts Sen. Elizabeth Warren, who challenged Biden during the primary and is now supporting his campaign.

Harris, a 55-year-old California senator and the daughter of Jamaican and Indian immigrants, addressed race and equality in a personal way Biden cannot when he formally accepts his party’s presidential nomination on Thursday.

“There is no vaccine for racism. We have got to do the work,” Harris said, her words emphatic though she was speaking in a largely empty arena near Biden’s Delaware home.

“We’ve got to do the work to fulfill the promise of equal justice under law,” she added. “None of us are free until all of us are free.”

Just 75 days before the election, Biden must energize the disparate factions that make up the modern Democratic Party — a coalition that spans generation, race and ideology. And this fall voters must deal with concerns over the COVID-19 pandemic that has created health risks for those who want to vote in person — and postal slowdowns for mail-in ballots, which Democrats blame on Trump.