Sen. Joe Biden has emerged as the favorite among Democrats to be Barack Obama's running mate for his understanding of foreign policy in grave global times and his fighting spirit against the Republicans.

Obama is keeping his decision quiet, but his staff in Chicago and party activists who await a decision are buzzing about Biden because he can address two of Obama's biggest weaknesses -- his lack of experience, especially on world affairs, and his reluctance to go on the attack. The speculation is less of an indicator of whether Biden will ultimately be Obama's pick, and more of a recognition of the challenges their candidate faces at this pivotal moment in the race.

Biden's junior colleague in the Senate, Thomas Carper, said, "I look at the aspects of our ticket -- where we're strongest and where we're most in need of help -- and Joe brings more to the ticket than any of the other people whose names are mentioned as finalists."

He said he learned in a "discussion that involved protection for Joe" that security surrounding Biden would be ratcheting up. A Biden spokeswoman said the senator was unaware of any extra security.

Obama plans to appear with his new running mate Saturday, with the pick announced via text message to supporters. Obama also is widely thought to be considering Govs. Kathleen Sebelius of Kansas and Tim Kaine of Virginia, and Sen. Evan Bayh of Indiana.

Biden dismissed speculation that he's the likely choice, telling reporters, "You guys have better things to do. I'm not the guy."

However, Carper said he understands Biden's chances to be serious. A person familiar with the process said Biden had a long private meeting with Obama within the past two weeks and has turned over financial and medical records for vetting. The source said Biden also has talked repeatedly with Eric Holder, a deputy attorney general in the Clinton administration who is helping run Obama's search.

Biden, 65, was first elected to represent Delaware in 1972, when Obama was 11 years old and half the people living in the United States today weren't born yet. He is a curious front-runner for running mate for a candidate who won the primary by arguing he would bring change to Washington.

Mark Fabiani, communications director for Al Gore's 2000 presidential bid, said of Obama: "To me he's ridden the change horse this far in politics, and if I'm him, I don't want to get off that horse. And I don't worry about shoring up my foreign policy credentials. ... I worry about projecting an image of change."

Biden is a charismatic and hard-charging campaigner with a compelling personal story -- his wife and daughter were killed in a car accident a few weeks after he was first elected, but two sons survived serious injuries in the same crash. Biden commuted home to Wilmington each day from the Senate to care for them. The oldest son, Beau, is now Delaware's attorney general and a National Guard member whose unit is being deployed to Iraq in October.

The Los Angeles Times and AP contributed to this report.