Joseph Robinette Biden Jr. was sworn in Wednesday as the 46th president of the United States, pledging to confront an array of convulsing challenges and bring healing and unity to a deeply fractured nation.

“This is our historic moment of crisis and challenge,” Biden said in an inaugural address that called on America to end its “uncivil war” and embrace a united front amid a series of daunting crises. “Unity is the path forward. And we must meet this moment as the United States of America. If we do that, I guarantee you we will not fail.”

With his hand on his thick family Bible and with his wife, Jill Biden, by his side, Biden recited the oath administered by Chief Justice John G. Roberts Jr. The moment marked the pinnacle of a career in public leadership that began a half-century ago.

Moments before, Kamala D. Harris took her oath of office, making her the country’s first female vice president, and also the first Black American and first person with Asian heritage to hold the nation’s second-highest office. She had placed her hand on twin Bibles, one from a family friend and the second belonging to Thurgood Marshall, the first African American justice of the Supreme Court.

Biden replaces outgoing president Donald Trump, whose ­scandal-plagued single term was constantly dogged by accusations that he failed to uphold his own oath — including in recent weeks, as he refused to concede the election, tried to browbeat his vice president, Mike Pence, into violating the Constitution and inspired a deadly attack by supporters on the U.S. Capitol.

Though Biden largely refrained from mentioning Trump by name Wednesday, he used his first day in office to tacitly repudiate the outgoing president — signing executive orders that reverse Trump administration measures affecting a range of issues, including the coronavirus and climate change. Biden signed a total of 17 executive orders and actions Wednesday, including a mask mandate for federal property, as well as moves to rejoin the Paris climate accords and World Health Organization, after Trump had withdrawn from those and other global forums during his presidency.

Biden did not wait long to begin staffing up his administration, swearing in top White House aides Wednesday in a low-key virtual ceremony and looking toward the Senate’s nighttime confirmation of his first Cabinet member — Director of National Intelligence Avril Haines.

Biden, who at 78 is the oldest man to be sworn in as president, secured the office by pledging to be the polar opposite of Trump — to cool tempers rather than inflame them. He has promised to undo much of Trump’s legacy and restore what he refers to as “the soul of America” by proving that the past four years represented an aberration rather than an enduring rift in the national fabric.