SPRINGFIELD, Ill. — Senator Obama introduced Senator Biden of Delaware today as a man “ready to step in and be president,” and the newly minted running mate quickly converted his debut on the Democratic ticket into a slashing attack on Republicans seeking four more years in the White House.

Senator McCain will have to “figure out which of the seven kitchen tables to sit at” when considering his own economic future, said Mr. Biden, jabbing at the man he called his personal friend.

It was a reference to Mr. McCain’s recent inartful admission — in a time of economic uncertainty — that he was not sure how many homes he owns.

Before a vast crowd spilling out from the front of the Old State Capitol, Mr. Obama said Mr. Biden was “what many others pretend to be — a statesman with sound judgment who doesn’t have to hide behind bluster to keep America strong.”

Democrats coalesced quickly around the 47-year-old Obama’s selection of the veteran of three decades in the Senate — a choice meant to provide foreign policy heft to the party’s ticket for the fall campaign against Mr. McCain and the Republicans.

Mr. Biden emerged as Mr. Obama’s choice after a secretive selection process that reviewed at least a half-dozen contenders — but evidently not Senator Clinton, the former first lady who was Mr. Obama’s tenacious rival across the primaries and caucuses of winter and spring.

Mr. Obama made a symbolic choice for the site of the ticket’s first joint appearance.

It was a brutally cold winter day more than a year ago when he stood outside the historic structure in the Illinois capital to launch his quest for the White House.

He returned this day in sunshine, the party’s improbable nominee-in-waiting, a black man in his first Senate term who outdistanced a crowded field of far better-known and more experienced rivals for the nomination.

The Democratic National Convention opens in Denver Monday to nominate him as president and Mr. Biden as vice president, the ticket that Democrats hope to ride into the White House after eight years of Republican rule.

A security fence sprung up overnight around the Pepsi Center as the pace of preparations picked up in advance of Monday night’s opening session, and police on bicycles patrolled nearby streets.

Inside the sports arena, even the Zamboni machine — the lumbering, wheeled vehicle used to resurface the ice between periods of hockey games — had been moved out to make room for the Democrats.

Mr. McCain’s convention opens on Labor Day. He has yet to select a running mate.

Polls indicate a highly competitive race at the end of a summer in which Mr. McCain eroded what had been Mr. Obama’s slender advantage in the national surveys.

Responding to Mr. Obama’s pick, the McCain campaign wasted no time trying to turn the selection to its own purposes.

It quickly produced a television ad featuring Mr. Biden’s previous praise for Mr. McCain and comments critical of Mr. Obama. In an ABC interview last year, Biden had said he stood by an earlier statement that Mr. Obama wasn’t yet ready to be president and “the presidency is not something that lends itself to on-the-job training.”

Mr. Obama brought Mr. Biden on stage with his glowing introduction to the strains of Bruce Springsteen’s “The Rising.”

The newly named running mate moved center stage in shirt-sleeves at a brisk trot that belied his 65 years, and embraced Mr. Obama.

“I’m glad to be here,” said the man who has twice sought the presidency and emerged as Mr. Obama’s pick only in the past few days.

Thousands of newly printed signs bearing the words Obama/Biden sprouted in the crowd that waited in anticipation in 90-degree temperatures.

Both men spoke for 16 minutes — unlikely a coincidence given Mr. Biden’s reputation for verbosity.

Mr. Obama’s remarks were carefully crafted to emphasize Biden’s accomplishments in the Senate, his blue-collar roots and — above all — his experience on foreign policy.

“I can tell you Joe Biden gets it,” he said. “He’s that unique public servant who is at home in a bar in Cedar Rapids and the corridors of the Capitol, in the VFW hall in Concord, and at the center of an international crisis,” he said.

In contrast to the Obamas and the McCains, the Delaware senator isn’t a multimillionaire. Mr. Biden and his wife, Jill, have $59,000 to $366,000 in assets and $140,000 to $365,000 in debts, including a $15,000 to $50,000 line of credit Mr. Biden co-signed with his son to cover college expenses, according to a June financial disclosure report, which describes assets and liabilities in ranges.

Mr. Obama recounted the personal tragedy that struck Mr. Biden more than 30 years ago, within days of his election to the Senate, when his first wife and their child were killed in an automobile accident.

He said Mr. Biden raised his surviving children as a single parent, commuting between the Capitol and Delaware daily on the Amtrak train.

“For decades, he has brought change to Washington, but Washington hasn’t changed him,” Mr. Obama said, attempting to blunt an emerging Republican line of attack that notes Mr. Biden’s three decades in the polished corridors of the Capitol.

“He’s an expert on foreign policy whose heart and values are rooted firmly in the middle class.”

In a jab at McCain that foretold Biden’s far sharper criticism, Mr. Obama said his political partner “will give us some real straight talk.”

Mr. Biden offered parts praise for Mr. Obama and criticism of Mr. McCain.

“You can’t change America and make things better for our senior citizens when you signed on to Bush’s scheme of privatizing Social Security,” he said.

“You can’t change America and end this war in Iraq when you declare — and again these are John’s words — ‘No one has supported President Bush in Iraq more than I have,’ end of quote. ladies and gentlemen, you can’t change America, you can’t change America when you know your first four years as president will look exactly like the last eight years of George Bush’s presidency.”

Mr. Biden returned to Delaware to prepare for the fall campaign. Mr. Obama was returning to Chicago before heading out to a trip across swing states en route to the convention.