SPRINGFIELD, Ill. -- Barack Obama's newly minted running mate will join the Democratic hopeful onstage Saturday at a rally in this capital city where Obama launched his White House bid, a campaign official said.

A senior Obama adviser told The Associated Press on condition of anonymity Tuesday that Obama and his veep choice will appear in front of the former state Capitol where Abraham Lincoln once served. The last time Obama appeared there, he announced he was running for president.

The disclosure narrowed the window Obama has to reveal his running mate. The list of possibilities is widely believed to be down to Delaware Sen. Joe Biden, Kansas Gov. Kathleen Sebelius, Indiana Sen. Evan Bayh and Virginia Gov. Tim Kaine, who planned to campaign with Obama Thursday in his home state.