"They have very few moments where they can completely control: the nomination and announcing -- this is one of those," Feldman said.
What is known is that when Obama is ready to make his selection public, staff at his campaign headquarters will send out the word by e-mail and text message to thousands of supporters as well as the media.
And Obama and his running mate will kick off a swing through battleground states with a rally Saturday in Springfield, Ill., the same spot where Obama announced that he was a candidate for president 18 months ago.
The political scrutiny has unearthed interesting details about the three Democrats waiting in the wings.
For instance, Kaine plays the harmonica. And despite the fact that Kaine and Obama have lived as far apart as possible and still be in America -- Hawaii and Virginia -- they both have origins in the middle of the country.
"My mom and my grandparents and his mother and grandparents both spent formative time in El Dorado, Kan.," Kaine has said.
If the ticket is Obama-Kaine, it would feature four Harvard Law grads, since Obama and Kaine, as well as their wives, are all Harvard Law alumni.
Obama and Bayh also have a few things in common.
Obama gave the convention keynote address in 2004, and Bayh gave the keynote in 1996. Bayh's father, Sen. Birch Bayh, D-Ind., made a presidential bid in 1976 under the slogan "Yes, we can." While it didn't work for Bayh's father, he's hoping it works for an Obama-Bayh ticket.
The Obamas have two children, as do the Bayhs, although the Bayh kids are twins.
Also, Bayh likes to run marathons, which is a good thing because this is the third time he's been in the running for the VP slot since 2000.
Biden doesn't have such striking things in common with Obama.
Obama likes basketball, while Biden is a weight lifter. But Biden, like Obama, has a history of overcoming problems. Once a stutterer, Biden is known these days as a nonstop yakker.
Biden has also had to overcome tragedy. His first wife and young daughter died in a car accident before he first took office more than 30 years ago.
"The only way you deal with those things, I think, you just got to focus on what's left, what you have," the Senate veteran said of the tragedy in an interview with ABC News anchor Charles Gibson.
While the consummate Washington insider, he's never lived a day in D.C., commuting each day via Amtrak from his home in Delaware.
The Republicans can afford to take a little more time with their veep decision, since the GOP convention begins Monday, Sept. 1, a week after the Democrats.
McCain reportedly intends to announce his choice Friday, Aug. 29, the day after the Democrats end their convention.
McCain supporters are calling around to top party donors and Republican delegates asking how they would feel if McCain picked a running mate who supports abortion rights, like former Pennsylvania governor and Homeland Security chief Tom Ridge.
The reaction from conservatives in the party has not been positive.