CHICAGO — Democrats coalesced around Senator Obama’s selection of Senator Biden as his running mate on Saturday while Republicans quickly seized on the Delaware senator’s past criticism of the presidential candidate’s inexperience.

As the newly minted ticket readied for its first joint appearance, in Springfield, Ill., Senator Clinton called Mr. Biden “an exceptionally strong, experienced leader and devoted public servant.” Mrs. Clinton, Mr. Obama’s most persistent rival through the primaries and the caucuses, was an also-ran in Mr. Obama’s vice presidential search.

Majority Leader Senator Reid said the Obama-Biden ticket will bring the change the country needs, including a filibuster-proof Senate majority.

Republican presidential candidate Senator McCain called Mr. Biden, his longtime Senate colleague and friend, to congratulate him, McCain campaign spokesman Tucker Bounds said. “It was a brief conversation. They’ve known each other for years,” he said.

Mr. McCain’s campaign wasted no time to try to turn the choice against Mr. Obama, producing an ad featuring Mr. Biden’s previous praise for Mr. McCain and comments critical of Mr. Obama from an ABC News interview last year. Mr. 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.”

“Biden has denounced Barack Obama’s poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing — that Barack Obama is not ready to be president,” a McCain campaign spokesman, Ben Porritt, said in a statement.

Some of Mr. Biden’s Republican colleagues in the Senate praised the Delaware Democrat, including Senator Lugar of Indiana. Senator Hagel called Mr. Biden “the right partner for Barack Obama” and the decision “good news for Obama and America.”

The Obama-Biden rally was set for old state Capitol where the Illinois senator kicked off his presidential campaign nearly 20 months ago. They were to be joined by Mr. Obama’s wife, Michelle; Mr. Biden’s wife, Jill; and the Bidens’ three adult children, Hunter, Beau, and Ashley.

A crowd of well-wishers waved to Mr. Biden and applauded as he left his home in Wilmington, Del., for a flight to Illinois just before noon EDT.

The Obama campaign sent a text message announcing his choice to supporters’ phones and e-mail addresses about 3 a.m. EDT, the latest innovation by a tech-savvy operation that has deftly used the Web as a fundraising and organizing tool. The rollout was diminished somewhat when word of Mr. Obama’s choice began leaking out to news media shortly before 1 a.m. EDT.

Mr. Obama and his famously disciplined team managed to keep Mr. Biden’s selection a secret for several days, leading to a frenzy of speculation that mounted throughout the week. Two of those thought to be on the short list — Governor Kaine and Senator Bayh — indicated Friday night that Mr. Obama had chosen someone else.

Mr. Biden, 65, is a creature of Washington, a 35-year Senate veteran and chairman of the Foreign Relations Committee whose national security credentials will help patch a hole in Mr. Obama’s relatively thin resume. Polls show that McCain holds a wide lead over Mr. Obama on the question of who is better prepared to be commander in chief.

Mr. Biden’s straighforward style and working-class Catholic roots in Scranton, Pa., were also expected to help Mr. Obama appeal to middle- and working-class voters in battleground states like Ohio and Pennsylvania who favored Clinton in the primaries.

Officials close to Mrs. Clinton said she was never formally vetted for the No. 2 position. The former first lady, who finished narrowly behind Mr. Obama in the primaries, will address the convention Tuesday night and her name will be placed in nomination even though she has endorsed Mr. Obama and has urged her delegates to support him.

“Senator Biden will be a purposeful and dynamic vice president who will help Senator Obama both win the presidency and govern this great country,” Mrs. Clinton said in the statement.

Mr. Biden has established a generally liberal voting record and a reputation as a long-winded orator. As a member of the Judiciary Committee — he was its chairman from 1987 to 1995 — he has played a key role in considering anti-crime legislation, Supreme Court nominees and constitutional issues.

While the war in Iraq has been supplanted as the campaign’s top issues by the economy in recent months, the recent Russian invasion of Georgia has returned foreign policy to the forefront.

Mr. Biden was elected to the Senate at the age of 29 in 1972, but personal tragedy struck before he could take office. His wife and their 13-month-old daughter, Naomi, were killed when a tractor-trailer broad-sided her station wagon. Mr. Biden took his oath of office for his first term at the hospital bedside of one of his sons.

Mr. Biden dropped out of the 2008 race for the Democratic presidential nomination after a poor finish in the Iowa caucuses, but not before he talked dismissively of joining someone else’s ticket.

He had stumbled on his first day in the race, apologizing for having described Mr. Obama as “clean.” Months later, Mr. Obama spoke up on Mr. Biden’s defense, praising him during a campaign debate for having worked for racial equality.

It was Mr. Biden’s second try for the White House. The first ended badly in 1988 when he was caught lifting lines from a speech by British Labor Party leader Neil Kinnock.

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Liz Sidoti reported from Washington. Associated Press writer Mike Glover in Sedona, Ariz., contributed to this report.