Democratic presidential nominee Joe Biden will seek to unite the American public around his candidacy Thursday, using his convention address to portray the country as resilient and hopeful after three days in which party luminaries lambasted President Trump.

Mr. Biden’s remarks—in which he will formally accept the Democratic presidential nomination, three decades after he first sought the job—will cap a first-of-its-kind virtual convention. His allies, including former President Barack Obama, have spent the week portraying Mr. Trump as failing to meet the challenges of the presidency and mismanaging the U.S. response to the coronavirus pandemic. They have depicted Mr. Biden as empathetic and thoughtful and someone who would be a more conventional leader of the country.

Mr. Biden will use his own speech to “lift up the strength of the American people and lay out a vision for how we will build back better and emerge from these crises stronger than ever before,” said Kate Bedingfield, Mr. Biden’s deputy campaign manager.

Mr. Biden said Wednesday during a meeting with the party’s Hispanic caucus that he intended to show how “the Democratic Party is coming together to unite our country, to stand up for working people and to deliver the promise of this nation to all Americans, particularly those who have been denied its full promise for much too long.”

Mr. Biden won’t have the raucous, confetti-covered crowd of a typical convention. He will deliver his address instead to a small group of mostly media at a Wilmington, Del., convention center after the pandemic prompted Democrats to cancel their planned gathering in Milwaukee.

The party has used the convention to showcase past party leaders, including Mr. Obama and former President Bill Clinton, and to spotlight what it sees as the next generation, chief among them Mr. Biden’s running mate, Sen. Kamala Harris of California.

The televised event has featured liberals in the party, led by Vermont Sen. Bernie Sanders, and Republicans such as former Ohio Gov. John Kasich to send a message that any Americans who are disenchanted with Mr. Trump are welcome in Mr. Biden’s coalition.

Mr. Obama, in his sharpest criticism of Mr. Trump to date, denounced his successor as someone who has shown “no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves.”

Ms. Harris, speaking from an auditorium designed to resemble the floor of past conventions and where Mr. Biden will also address the nation, said Mr. Trump has careened from crisis to crisis.

“Right now, we have a president who turns our tragedies into political weapons,” Ms. Harris said. “Joe will be a president who turns our challenges into purpose.”

Mr. Trump this week has dismissed the criticism of him, and his campaign has said voters will reject a Democratic Party that has moved to the left on some issues. Republicans will hold a small convention gathering Monday in Charlotte, N.C., followed by televised events during the week and capped by an Aug. 27 speech in which Mr. Trump will accept the party’s nomination at the White House.

Mr. Trump also planned to deliver a dose of counter-messaging Thursday near Scranton, Pa., the city where Mr. Biden was born and spent part of his childhood.

National polls during the summer have shown Mr. Biden with an edge over the president and a lead in several crucial battleground states such as Pennsylvania, Michigan and Florida.

Mr. Biden’s team has sought to portray the former vice president as a family man who has weathered personal tragedies and can relate to Americans suffering during the pandemic. They have reminded voters of his time as a single father who commuted home from the Senate aboard an Amtrak train when his sons were children.

Dan Schwerin, a former speechwriter for Hillary Clinton’s 2016 campaign, said Mr. Biden could use the fact that he will be speaking to viewers on television, rather than to a noisy crowd, to deliver a more personal message.

“That may well be more effective than a traditional crowd-pleaser ever would have been,” Mr. Schwerin said by email.

Mr. Biden launched his third bid for the presidency in April 2019, declaring the election a “battle for the soul of this nation.” He joined a sprawling Democratic field as an early front-runner, bolstered by strong name recognition after serving eight years as Mr. Obama’s vice president and 36 years as a U.S. senator from Delaware.

As the Democratic primary unfolded, an ideological battle between moderates and the party’s progressive wing often placed Mr. Biden at the center of attacks in the debates, with rivals from his political left criticizing his record on issues such as immigration and criminal justice.

Mr. Biden defended the administration in which he served, branding himself an “Obama-Biden Democrat,” and clashed with progressives such as Mr. Sanders and Sen. Elizabeth Warren over the costs of their proposed single-payer health-care plans.

Mr. Biden’s standing was diminished with poor showings in both Iowa and New Hampshire, the first two nominating contests, where he placed fourth and fifth, while Mr. Sanders was propelled to the top of the field.

Mr. Biden came in a distant second to Mr. Sanders in Nevada, but he turned his campaign around in South Carolina’s primary. His strong support from Black voters and the endorsement of Rep. James Clyburn (D., S.C.), the most influential Democrat in the state, helped resurrect his bid.

Other moderate candidates quickly dropped out of the race and endorsed Mr. Biden ahead of Super Tuesday, and he notched a string of victories with one-third of the party’s pledged delegates up for grabs.

As the former vice president extended his lead in a two-man race with Mr. Sanders, the coronavirus pandemic brought the campaign to a standstill. Mr. Sanders, whose path to victory had significantly narrowed, dropped out of the race in April and endorsed Mr. Biden, effectively making him the presumptive nominee.

Mr. Biden has spent recent months largely holding virtual events, where he has sharply criticized Mr. Trump’s handling of a pandemic that has claimed more than 170,000 American lives.