A senior Obama adviser told The Associated Press that Obama and his vice presidential 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 senator Joe Biden, Kansas governor Kathleen Sebelius, Indiana senator Evan Bayh and Virginia governor Tim Kaine, who plans to campaign with Obama in his home state on Thursday.

Obama's major rival for the nomination, Hillary Clinton, was seen by some Democrats as a long-shot.

As he drew closer to naming a running mate in the final days before the Democratic convention, Obama issued a blunt counterattack against what he contends is Republican John McCain's habit of questioning his "character and patriotism".

Vice presidential picks have seldom been as important as this year. Obama was thought to be looking for a running mate who adds weight to the Democratic ticket, given the Illinois senator's brief tenure on the national political scene.

The choice is equally vital for McCain, who turns 72 on August 29 and would be the oldest first-term American president.

The Arizona senator is reportedly considering making his vice presidential announcement immediately after next week's Democratic convention in a bid to diminish Obama's post-convention glow.