WASHINGTON --- Joe Biden has lived a life of second chances, a cycle that's been cruel and redemptive by turns. Now he's starting over once again.

Deeply private yet in-your-face, collegial yet ideological, the Delaware senator brings a wealth of foreign policy experience to Barack Obama's Democratic ticket, plus wisdom in the ways of Washington and an infectious enthusiasm for political donnybrooks.

He adds suspense, too, over the question of when -- not if -- he'll put his foot in his mouth. Mr. Biden's agile mind comes with a loose tongue that can't always be properly restrained.

In Scranton, Pa., Mr. Biden's Catholic schoolmates nicknamed him Dash because he stuttered so much his speech sounded like Morse code. Mr. Biden overcame that rip at his confidence, smoothed his talk and doesn't seem to have quieted down since.

He came to Washington as a wunderkind, elected to the Senate in 1972 at age 29 -- the earliest possible age -- and just meeting the rule that one must be 30 when sworn in. The knock against him used to be that he was more sizzle than steak, articulate but perhaps not all that deep.

At age 65, as a party elder and veteran of titanic judicial nomination struggles, world crises and legislative deal-making, that rap has faded.

His hearings as chairman of the Senate Foreign Relations Committee are historical soliloquies on the fly, complete with grace notes liberally dispensed to colleagues and witnesses of any political persuasion, humor often directed at himself and sharp-tongued fulmination over what he sees as the failures of the Bush administration.

His first presidential campaign, in 1987, was a "train wreck" by his own description, one of those times that forced him to pick up pieces and start anew.

He'd lifted lines from a British politician, exaggerated his academic achievements when boasting to a voter who challenged him ("I have a much higher IQ than you do, I suspect," Mr. Biden recalls saying) and suffered horrendous headaches that turned out to be life-threatening brain aneurisms that kept him out of the Senate for seven months.

"In the aftermath I had to remake my health, my reputation, and my career in the Senate," he writes in his memoirs. And that was not the worst of his shattering episodes -- not even close.

On Dec. 18, 1972, five weeks after Mr. Biden was elected to the Senate, his wife, Neilia; infant daughter Naomi ;and sons Beau and Hunt; were out getting a Christmas tree when a tractor-trailer broad-sided their car. Mr. Biden was in Washington.

Neilia and Naomi died, and the boys were critically injured.

He devoted himself to the care of his sons and was sworn in at the bedside of one of them before they both recovered fully, growing up to become lawyers. In 1977, Mr. Biden married Jill Tracy Jacobs. They have a daughter, Ashley.

He still will not work Dec. 18, the date of the accident.

Mr. Biden does not talk often of the tragedy but decades later, anything to do with the welfare of his children still rankles -- and explains perhaps his sharpest rebuke of Mr. Obama and Hillary Rodham Clinton in this year's primary campaign.

Capt. Beau Biden, a member of the Delaware National Guard and the state's attorney general, had been preparing for deployment to Iraq, and it did not sit well with his father that Mr. Obama and Mrs. Clinton had at times voted against money for the war.

"There's no political point worth my son's life," Mr. Biden snapped. "There's no political point worth anybody's life out there. None."

Mr. Obama and Mr. Biden have taken opposing positions in Senate votes at least a dozen times.

Mr. Obama voted for tougher fuel economy standards and an energy bill both opposed by Mr. Biden. Mr. Obama endorsed the Bush administration's military procedures for detaining and prosecuting foreign terrorism suspects at Guantanamo in a bill Mr. Biden voted against.

The two were also at odds over legislation making it harder for people to erase debts in bankruptcy; Mr. Biden supported it and Mr. Obama was opposed. Also unlike Mr. Obama, Mr. Biden supported stricter rules on lawmakers' pet projects, the confirmation of Gen. Michael Hayden as CIA chief and renewal of the Patriot Act.