Obama, whose public swearing-in ceremony will be held Monday at the Capitol building, took the oath of office in the grand Blue Room of the White House, his hand on a family Bible held by his wife, Michelle, as the couple's two daughters looked on.
The oath was administered by John Roberts, chief justice of the U.S. Supreme Court, who famously flubbed the oath four years ago when he swore in the country's first African-American president. There were no such hitches on Sunday.
"I did it," Obama told his daughters cheerfully after taking the oath. Eleven-year-old Sasha Obama replied: "You didn't mess up."
The U.S. Constitution mandates that presidential terms begin on Jan. 20. It's customary that when the day falls on a Sunday, the public swearing-in is held the following day — this year, it's falling on the national holiday devoted to revered civil rights leader Martin Luther King Jr.
Obama begins his next four years in office amid far different circumstances than he did in 2009, when his soaring messages of hope and change dared millions of Americans to dream that their country could be the better, brighter place that King himself envisioned decades earlier.
And yet the commander-in-chief now presides over a bitterly divided U.S. Congress, although the dramatically changing face of America, in particular the growing number of Hispanic voters, helped to decisively propel him to a second victory in November.
The president's first term kicked off as a devastating economic recession was taking hold, thwarting some of his loftiest legislative goals and compelling his administration to spend billions in efforts to ward off a full-fledged depression.
It's also been marked by nasty partisan brawling incited largely by a Republican opposition that made no bones about its determination to put the brakes to Obama's legislative agenda at every turn. They also vowed to deny him a second term, an ultimately fruitless mission.
There have been successes — the president managed to do what so many in the Oval Office had failed to achieve, passing a sweeping overhaul to the country's health-care system that provided health insurance to millions of Americans. He also authorized the successful Navy SEALs raid that finally captured and killed Osama bin Laden, the al-Qaida mastermind behind 9-11.
There were failures — Obama did next to nothing on immigration reform, failed to close the Guantanamo Bay military prison as promised and was utterly unable to make deals with John Boehner, the Republican speaker of the House of Representatives, on taxes, spending cuts, the debt ceiling and the so-called fiscal cliff.
And there were tragedies, most notably a series of horrific mass shootings — one that almost killed a Democratic congresswoman, another that left 12 dead in a Colorado movie theatre and perhaps the most heinous of all, the massacre of 20 young schoolchildren in small-town Connecticut last month.
Indeed, the carnage in Newtown, Conn., at the hands of a troubled young man toting his mother's assault rifle shook the president to his core. It's resulted in a renewed White House push for gun control that is likely to be a hallmark of Obama's second term.
Even as inauguration festivities played out across the U.S. capital all weekend, gun rights activists held a Gun Appreciation Day as a form of protest against Obama's proposals to combat gun violence. Five people were injured Saturday at gun shows in three states in accidental shootings.
After four difficult years of governing, a greyer, more sombre Obama will address a crowd of as many as 800,000 people on Monday.
One of the president's closest advisers, David Plouffe, pointed out Sunday that despite the partisan warfare on Capitol Hill, polls suggest most Americans back Obama's agenda.
"There's vast support out there for balanced deficit reaction, investments in education and manufacturing, immigration reform, gun safety," he said on CBS's "Face The Nation."
"So on the issues the president intends to really push and focus on, there's massive support in the country, even amongst Republicans."
He added, however, that Obama intends to "do a better job" in his second term working with Congress. Plouffe also touched on something the president will likely underscore in his second inaugural address: the need to "make sure the American people are more connected to what's going on here."
As Obama unveiled his gun control proposals late last week, he urged Americans to say "enough" and demand action from their congressional representatives on gun violence.
In addition to gun control, the next four years will also include a major push for immigration reform from the White House. The Obama administration wants a sweeping bill that would include a path toward citizenship for millions of illegal immigrants already living stateside.
Obama will also face continuing pressure to bring the country's US$16 trillion national debt to heel. There will also be a decision in the weeks to come on the fate of TransCanada's Keystone XL pipeline.
There are myriad issues confronting Obama on the international front.
Israel continues to pressure the White House to launch military action against Iran in order to stop it from building nuclear weapons. There's also an ongoing civil war in Syria, increasingly tense U.S.-Russia relations, hostilities between Israelis and Palestinians and trade disputes with China.
Among the hottest tickets in the U.S. capital is Canada's embassy at 501 Pennsylvania Avenue, just a stone's throw from the Capitol with a stunning view of the action from its sixth-floor rooftop patio.
More than 1,500 people will be in attendance at the party hosted by Ambassador Gary Doer. They include Foreign Affairs Minister John Baird and some American VIPs.
The guests will be treated to beaver tails, Tim Hortons coffee and Crown Royal cocktails among other Canadian fare.
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