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federal property, as well as moves to rejoin the Paris climate accords and World Health Organization, after Trump had withdrawn from those and other global forums during his presidency.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.

Four years after Trump delivered a dark and abrasive inaugural address pledging to end “American carnage,” Biden took office seeking to appeal to the country’s more hopeful sentiments and make a plea for unity.

“Politics doesn’t have to be a raging fire, destroying everything in its path,” he said in his speech, calling on the nation to “start afresh.” The 21-minute speech hewed closely to the themes that shaped Biden’s presidential run, with several references to unity, comity and a restoration of American leadership on the global stage.

He cited his inauguration as a symbol of the country turning the page after its very democracy was tested as never before. Seated behind him as he spoke were three former presidents: Barack Obama and Bill Clinton, both Democrats, and Republican George W. Bush. After taking their oaths, Biden and Harris appeared alongside those former presidents in a bipartisan tribute at the Tomb of the Unknowns in Arlington, Va.

Trump, who did not participate in the inaugural events, was not present.

Two weeks earlier, the platform on which Biden was sworn in, at the West Front of the Capitol, was overrun by a mob that, incited by an incendiary Trump speech, stormed the House and Senate chambers trying to stop legislators from affirming Biden’s election victory. The decision by Biden and Harris to still take their oaths of office outside and at the Capitol was itself an act of defiance in the aftermath of the hours-long riot, during which Trump supporters screamed their intention to physically assault top elected officials.

“We’ve learned again that democracy is precious, democracy is fragile — and at this hour, my friends, democracy has prevailed,” Biden said during his speech, which made several references to the unprecedented chaos of recent weeks.

Biden promised to confront and defeat the scourge of white supremacy and domestic terrorism, challenges that have taken on fresh urgency in the wake of the attempted insurrection.

“A cry for racial justice, some 400 years in the making, moves us,” he said. “The dream of justice for all will be deferred no longer. The cry for survival comes from planet itself, a cry that can’t be any more desperate or any more clear. And now a rise of political extremism, white supremacy, domestic terrorism that we must confront and we will defeat.”

In an emphatic denunciation of the “riotous mob” that tried to block him from becoming president, Biden declared that his inauguration Wednesday at the Capitol was proof that the effort to undermine democracy had failed.

“It did not happen. It will never happen,” he said. “Not today, not tomorrow, not ever. Not ever.”

While Biden described America as a “place of hope and light, of limitless possibilities,” during an emotional goodbye ceremony in Delaware on Tuesday, the scene surrounding his inauguration in Washington on Wednesday offered a visceral reminder of the dark challenges he now faces.

The nation’s capital remained in a state of partial lockdown, with more than 20,000 members of the National Guard patrolling the streets to prevent a repeat of the attempted Jan. 6 insurrection that left one police officer and four rioters dead.

The National Mall that Biden and Harris stood before as they took their oaths was filled not with people but with 200,000 flags, a reflection of the deadly pandemic that is still spiraling nationwide a year after Trump first asserted it was “totally under control.”

Later Wednesday, Biden and Harris walked down a short segment of Pennsylvania Avenue in front of the White House in a scaled-back version of the traditional parade route. While in-person crowds were largely prohibited — a reflection of the enduring impact of the crisis — a virtual program featuring musicians, actors and other celebrities, organizations and groups streamed online.

More than 400,000 Americans have died of covid-19, and the viral pathogen continues to upend life for millions. Key to Biden’s success will be his ability to contain the pandemic and marshal the resources to hasten distribution of vaccines.

Trump, who recently became the first president in American history to be impeached twice, became the first incumbent to refuse to attend his replacement’s oath-taking since 1869. And he has continued to claim falsely that he was the rightful victor in November’s election.

Trump’s falsehoods have helped convince large swaths of the country that Biden is an illegitimate president, effectively stunting his political capital from the outset of his term.

Spurning the traditional handoff, Trump left the White House early Wednesday, still having not spoken to Biden since their last pre-election debate. Shortly before boarding Air Force One at Joint Base Andrews in Maryland, Trump lauded what he said were his accomplishments and asserted that he had delivered the incoming administration — he did not mention Biden by name — “the foundation to do something really spectacular.”

“We will be back in some form,” he said. “So have a good life. We will see you soon.”

The recent turmoil added to the multiple crises Biden faces: In addition to the pandemic, the country’s economy is buckling, and the incoming president has pledged to address climate change and racial justice — two issues that dominated the past year, with huge climate-propelled fires in California and massive unrest in cities and towns over police violence.

“Coming in he’s got his hands full, obviously,” said Christopher J. Dodd, a former senator from Connecticut and a close Biden confidant. “Any one of these issues alone would be a major problem. But combining all of them poses some real challenges.”

Dodd stressed that Biden’s age and his deep experience with national politics and crises over the past half-century will guide him. “There is this expression, ‘It’s not his first rodeo,’ ” Dodd said. “There is a wonderful maturity and calm that Joe has demonstrated over these past few months. That’s the voice of experience, of confidence, of discipline — you don’t become an alarmist. The country wants to see in their leader confidence and assurance.”

Biden, a six-term senator before his eight years as vice president, has been trying to court members of Congress as he prepares to pitch an ambitious legislative agenda that includes economic relief tied to the pandemic, an immigration overhaul, infrastructure spending and climate-change initiatives.

Biden’s Democratic Party has taken control of the White House and Congress for the first time since 2011, yet it holds only a narrow advantage in the House and the barest majority in the Senate — a 50-50 split driven by two Democratic runoff wins in December that will leave Vice President Harris to break any ties.

Biden, the second Catholic to be elected president, after John F. Kennedy, invited Republican and Democratic congressional leaders to attend Mass with him Wednesday morning ahead of the inauguration. The decision by Senate Majority Leader Mitch McConnell (R-Ky.) and House Minority Leader Kevin McCarthy (R-Calif.) to attend — and to skip a farewell event for Trump that took place around the same time — offered one sign that Biden’s approach may at least take some of the vitriol out of the policymaking process.

But McConnell, who spent years as minority leader frustrating then-President Obama’s legislative agenda when Biden was vice president, has not openly embraced the idea of helping the new president pass legislation.

Several prominent GOP lawmakers, including McCarthy, have cast doubt on Biden’s legitimacy — voting earlier this month to challenge the results of the election and endorsing Trump’s specious fraud claims.

Biden has said he would not question the motives of his political opponents, pledging to find common ground and pass bipartisan legislation despite Washington’s bitter polarization. The theme of his inaugural was “America United.”

While Biden did not mention Trump by name during his speech, he called out those who peddle misinformation and push “lies told for power and for profit.”

“We must reject the culture in which facts themselves are manipulated and even manufactured,” he said to applause.

Biden entered the White House later Wednesday and told reporters that there was “no time to waste” before signing a stack of executive actions. They included a repeal of Trump’s ban on U.S. entry for citizens of some majority-Muslim countries and a fortification of the Deferred Action for Childhood Arrivals program protecting millions of young immigrants from deportation. Trump had tried to end the Obama-era program known as DACA.

After signing some of the orders, Biden told reporters that Trump had left him a “very generous letter” in the Oval Office, though he declined to detail its contents. The new president wore a mask, in contrast with the typical practice of his predecessor.

Biden bid an emotional farewell to his home state of Delaware on Tuesday, with tears streaming down his face as he thanked a group that included longtime friends for sticking with him through his circuitous and at times tragedy-laden journey from a middle-class childhood in Scranton, Pa., and Wilmington to 1600 Pennsylvania Ave.

“You’ve been there for us in good and bad and never walked away,” said Biden, whose failed bids for the White House in 1987 and 2008 make him one of the longest seekers of the presidency to actually obtain it. “And I am proud, proud, proud, proud to be a son of Delaware.”

As he left Delaware, where his first Senate election and the tragic death of his wife and daughter in a car accident occurred in quick succession 48 years ago, he sounded a note of optimism.

“Anything’s possible,” Biden said. “Anything’s possible in this country.”

Harris, 56, the daughter of Jamaican and Indian immigrants and a graduate of Howard University, is set to play a key role in the administration, not least as the potential tiebreaker in the Senate. She was administered the oath by Supreme Court Justice Sonia Sotomayor, the first Hispanic woman to serve on the nation’s highest court.

Harris’s ascension is in keeping with Biden’s pledge to assemble an administration that “looks like America.” Cabinet nominees on hand for the inauguration, several of whom would be the first of their respective backgrounds to serve in their posts, reflected Biden’s promise to have the most diverse administration in American history. The broad tapestry of races, genders and cultures surrounding Biden stood in sharp contrast with the Trump administration’s open rejection of diversity and multiculturalism.

But how much Biden is able to achieve with razor-thin Democratic majorities in Congress could depend on his ability to persuade Republicans to turn the page from the combative approach championed by Trump.

The former president seems to have no intention of making his successor’s job easier.

Since losing the election, Trump has raised more than $200 million for a new leadership PAC, publicly mused about running for president in 2024 and been banned from major social media sites for inspiring his followers to violently object to Biden’s presidency.

Even Trump’s pending impeachment trial could upend Biden’s legislative agenda, eating up critical time in the Senate that could otherwise be used to confirm nominees and mark up legislation.

While much of Trump’s legacy can be undone with a stroke of Biden’s pen, his sway over the Republican Party remains rock solid. A Washington Post-ABC News poll published this week found that 57 percent of Republicans wanted the party’s leaders to follow Trump’s lead rather than chart a different path. In that same poll, 70 percent of Republicans said they did not think Biden legitimately won the November election, a view not shared by Democrats and independents.

In a 20-minute video message released Tuesday, Trump touted his accomplishments and claimed to have “built the greatest political movement in the history of our country.”

Biden, who has said he decided to run for president in 2017 after hundreds of Trump supporters held a deadly white-supremacist rally in Charlottesville, now confronts the ramifications of Trump’s ability to amass a tenuous coalition of traditional Republicans, conspiracy theorists, racists and militant extremists.

While their alliance remains fractious, the loosely aligned groups have unified around the goal of opposing Biden’s agenda for the country.

Biden also must lead a diverse and anxious Democratic Party with differing opinions about the wisdom of his centrist approach. The energized liberal wing of the party is calling on the new president to take full advantage of unified power in Washington rather than trying to work with Republicans who are intent on obstructing his agenda.

A group of liberal organizations including the Justice Democrats published a memo Monday calling on Biden to pressure the party’s congressional leaders to get rid of the Senate filibuster and pass major legislation with a simple majority.

“Biden was elected with a mandate to break gridlock and deliver results. He should use it,” the memo said. “Just do it — reform the filibuster and deliver results to the American people.”

But the former vice president maintained his pitch to the opposing party even as it sought to delegitimize his presidency before it began.

“We must end this uncivil war that pits red against blue, rural versus urban, conservative versus liberal,” he said Wednesday, adding: “My fellow Americans, in the work ahead of us, we’re going to need each other. We need all our strength to persevere through this dark winter.”