Senator Obama says he has made a decision about a running mate, and the announcement will come first by way of a text message to supporters, reflecting his campaign's groundbreaking use of cutting edge technology and networking.
Speculation from Democrats and political analysts has focused on a small group of potential vice presidential picks. They include Senators Joe Biden of Delaware and Evan Bayh of Indiana, as well as Governors Tim Kaine of Virginia and Kathleen Sebelius of Kansas. The Associated Press reported that Texas Congressman Chet Edwards was also a surprise contender.
There was little speculation that Obama might pick Senator Hillary Clinton of New York, his rival in the Democratic primaries. Clinton has been given a key speaking role at next week's Democratic Convention.
Obama spoke about the qualities he is seeking in a running mate on CBS television's The Early Show.
"Obviously, the most important question is, is this person prepared to be president? The second most important question from my perspective is can this person help me govern," he said.
Obama also said he wants a vice president who will be independent.
"I want somebody who is going to be able to challenge my thinking and not simply be a 'yes person' when it comes to policy," he said.
Much of the speculation is focused on Senator Biden, chairman of the Senate Foreign Relations Committee. Many political experts say Biden would fit Obama's need for an experienced foreign policy hand on the Democratic ticket.
The selection of a running mate will come just days before Monday's opening of the Democratic National Convention in Denver, Colorado. Obama will be formally nominated at the four-day convention and will give his acceptance speech before a crowd of 75,000 people at a football stadium next Thursday.
Obama's Republican opponent, Senator John McCain, will be formally nominated at his party's national convention the following week in Minneapolis-St. Paul, Minnesota. McCain is expected to announce his vice presidential running mate shortly before the Republican convention.