WASHINGTON — Senator Obama selected Senator Biden late Friday night to be his vice presidential running mate, according to a Democratic official, balancing his ticket with an older congressional veteran well-versed in foreign and defense issues.

Mr. Biden, who has twice sought the White House, is a Catholic with blue-collar roots, a generally liberal voting record and a reputation as a long-winded orator.

Across more than 30 years in the Senate, he has served at various times not only as chairman of the Foreign Relations Committee, but also as head of the Judiciary Committee, with its jurisdiction over anti-crime legislation and Constitutional issues.

The official who spoke did so on condition of anonymity, preferring not to pre-empt a text-message announcement the Obama campaign promised for Saturday morning.

Mr. Obama’s campaign arranged a debut for the newly minted ticket on Saturday outside the Old State Capitol in Springfield, Ill.

The Democratic National Convention meets next week in Denver to hand Mr. Obama his long-sought presidential nomination, and then confirm Mr. Biden.

Mr. Biden slowly emerged as Mr. Obama’s choice across a long day and night of political suspense as other contenders gradually fell away.

First Governor Kaine let it be known that he had been ruled out. Then came word that Senator Bayh of Indiana had also been passed over.

Several aides to Senator Clinton — who was Mr. Obama’s closest rival for the presidential nomination — said they believed she also was out of contention. They added the Obama campaign had never requested financial or other records from her.

Despite passing over Mrs. Clinton, Mr. Obama has gone to great lengths to gain the confidence of her primary voters, agreeing to allow her name to be placed in nomination and permitting a roll call vote.

Other finalists in the veep sweepstakes were Governor Sebelius and Representative Chet Edwards.

Among those on the short list, Mr. Biden brought the most experience in defense or foreign policy — areas in which Mr. Obama is rated relatively poorly in the polls compared with Senator McCain.

A native of Scranton, Pa., Mr. Biden also has working-class roots that could benefit Mr. Obama, who lost the blue-collar vote to Mrs. Clinton during their competition for the presidential nomination.

Mr. Biden, 65, was elected to the Senate at the age of 29 in 1973.

Mr. Biden spent the day at his home in Delaware with friends and family. The normally loquacious lawmaker maintained a low profile as associates said they believed — but did not know — he would be tapped. They added they had been asked to stand by in case their help was needed.