WASHINGTON — Joe Biden, whose foreign policy expertise, Washington savvy and personal style earned him Barack Obama's trust and faith, was sworn in today as the 47th vice president of the United States.

With the world watching, Biden raised his right hand and took his oath of office from John Paul Stevens, associate justice of the Supreme Court. The longtime Delaware senator is now a heartbeat from the presidency, a counselor to Obama, and one of the government's key ambassadors to other global powers.

In traditional form, Biden swore to defend the Constitution against all enemies with true allegiance to the cause, to take on the obligation freely, and to faithfully discharge the duties of the office.

"So help me God," Biden said, to an outpouring of cheers from the Capitol platform and crowds below.

Biden shared a few thankful moments with his wife, Jill, and other members of his immediate family. He shared a handshake with Obama shortly before the president-elect took his own oath of office.

The vice president's most vital role is to assume the presidency in the event Obama cannot serve. Beyond that, his duties can be limited or expansive based on the wishes of his boss, the president.

Biden says he plans to loyally support Obama but also wants an active role in decision-making.

The 66-year-old Biden served 36 years as a Delaware senator.

He and his wife began Inauguration Day by attending a private service at St. John's Episcopal Church with Obama and the incoming first lady, Michelle Obama. The four then headed to the White House for coffee with President George W. Bush and first lady Laura Bush, Vice President Dick Cheney and his wife, Lynne, and leaders from Congress.

Elections are not built around or decided by running mates, and given the enormity and the history of the moment surrounding Obama today, Biden will retain second billing.

But his ascendancy to the vice presidency is a major turning point for the country too as Biden replaces Cheney, who assumed huge powers under Bush.

Cheney was a major voice on war and harsh interrogation techniques of suspected terrorists, and insists the Bush administration kept the nation safe; Biden accused him of doing more harm than any elected official in recent memory in "shredding the Constitution."

All that will be quickly be history, but Biden expects to play his own active role.

"This is a partnership," Biden said Monday in a taping of "The Oprah Winfrey Show."

"He's president of the United States, but as I said to him when he asked me, I said, `Barack, don't ask me unless the reason you're asking me is you're asking me for my judgment," Biden said. "'I get to be the last guy in the room when you make every important decision. You're president. Any decision you make, I will back."'

Biden ran for the presidency twice, in 1988 and in 2008, but never came close. He has served 36 years as a senator from Delaware, long establishing himself as a respected voice on foreign affairs and national security. He is well traveled and knows world leaders.

His wife, Jill, slipped on Winfrey's TV show by saying that her husband had been given a choice of being Obama's running mate or secretary of state; the latter nomination went to Hillary Rodham Clinton. A Biden spokeswoman later said Obama only offered Biden the vice president's job.

A lawyer by trade, Biden won election to the Senate at a young age and has spent most of his life working there, but has still made a daily train commute home to Wilmington, Del.

The vice presidency would seem to cap a career in which Biden has become a Democratic Party elder and a veteran of judicial nomination struggles, world crises and legislative dealmaking.

Born in working-class Scranton, Pa., Biden often speaks of a blue-collar work ethic. He did so Sunday at a grand inaugural celebration at the Lincoln Memorial, when Biden said people deserve a chance to work hard, have dignity and show thanks to earlier generations.

Biden often takes a ribbing for his tendency to be long-winded and get himself into trouble with verbal gaffes. "Rhetorical flourishes" is the way Obama describes them.