CHICAGO -- Democratic presidential candidate Barack Obama planned a typical day of campaigning Tuesday while speculation flared over when he would announce his choice of running mate.

Veep Stakes: Obama | McCain

Obama's campaign schedule is open after he concludes a bus tour of North Carolina and Virginia on Wednesday, leaving free the end of the week for an announcement. The Democratic National Convention is set to begin next Monday.

The list of potential running mates is widely believed to be down to four names: Virginia Gov. Tim Kaine, Delaware Sen. Joe Biden, Indiana Sen. Evan Bayh and Kansas Gov. Kathleen Sebelius. Obama's major rival for the nomination, Sen. Hillary Rodham Clinton of New York, was seen by some Democrats as a longshot pick.

Only Obama, his wife, Michelle, a handful of his most senior advisers and his two-member search committee know for certain who has been vetted and discussed. Staffers were already in place to aid Obama's pick, including more than a dozen seasoned operatives who have set up shop in a section of the campaign's Chicago headquarters.

Campaign manager David Plouffe has said that Obama supporters would receive first word of Obama's decision through a mass text message and e-mail, but otherwise the team has revealed little about what to expect. Historically, presidential tickets then tour battleground states to maximize media exposure, and Obama is expected to do the same.

Obama tapped a three-person team, including Caroline Kennedy, to help lead the vetting of a prospective vice presidential candidate.

Kennedy has begun managing the process with high-profile Democratic insiders Jim Johnson and Eric Holder. Johnson is the former CEO of Fannie Mae who also oversaw the vice presidential vetting for Sen. John Kerry in 2004 and Walter Mondale in 1984. Holder was deputy attorney general in the Clinton administration and has been a senior legal adviser to Obama's campaign.

A similar decision looms for Republican candidate John McCain. In hopes of grabbing the post-convention spotlight from Obama, McCain is considering naming his running mate in the few days between the time when Democrats leave Denver and when the Republicans begin their nominating convention in St. Paul, Minn.

McCain's top contenders are said to include Minnesota Gov. Tim Pawlenty and former Massachusetts Gov. Mitt Romney. Less traditional choices include former Pennsylvania Gov. Tom Ridge, an abortion-rights supporter, and Connecticut Sen. Joe Lieberman, the Democratic vice presidential pick in 2000 who now is an independent.