Obama's deliberations remain remarkably closely held. Aides said perhaps a half-dozen advisers were involved in the final discussions in an effort to enforce a command that Obama issued to staff: that his decision not leak out until supporters are notified.

Obama had not notified his choice -- or any of those not selected -- of his decision as of late Monday, advisers said. Going into the final days, Obama was said to be focused on three candidates: Sen. Evan Bayh of Indiana, Gov. Tim Kaine of Virginia and Sen. Joseph R. Biden Jr. of Delaware.

Some Democrats said they still hoped that he would choose Sen. Hillary Clinton of New York or Gov. Kathleen Sebelius of Kansas.

By all indications, Obama remains likely to chose someone relatively safe and avoid taking a chance with a game-changing selection. A similar strategic choice awaits Obama's Republican opponent, Sen. John McCain, who has been under pressure from some in his party to make a more daring choice.

Obama's advisers said he reached his decision while on vacation in Hawaii. They said it marked the end of what proved to be an unexpectedly intense process, condensed because he did not want to start actively vetting potential running mates before Clinton quit the race in June.

By contrast, McCain, who had wrapped up the Republican nomination months earlier, began his process in late spring.

Obama's team of lawyers had less time to review candidates, and several Democrats said it appeared that the number of candidates who were deeply vetted was limited to about a half dozen.

The team of advance workers and aides involved in planning the rollout -- timed to galvanize Democratic voters as Obama heads to Denver next week for the party convention -- have not been told whom Obama will select.

If all goes according to plan, the announcement will be made with text and e-mail messages to supporters early in the morning, in time to capture coverage on the morning news shows and take advantage of a full day's news cycle.

Obama and his new running mate then will begin a cross-country tour. Current plans call for them to be on the trail together for most of the time between the day of the announcement and the day Obama arrives in Denver, a week from Wednesday.

Obama's schedule calls for him to be in Virginia by Wednesday.

Aides said the announcement will come between Wednesday morning and Friday.

The rampant speculation during the selection process encompassed many of the best-known names in the party. The big choice for Obama, advisers said, was the extent to which he needed to choose someone who would fill perceived holes in his resume -- lack of experience, particularly in foreign policy -- versus a candidate who would reinforce his promise of change or one that might help him win a contested state.

Biden, the chairman of the Senate Foreign Relations Committee, falls into the category of helping Obama on foreign policy.

Kaine is a relative newcomer to national politics and would reinforce the notion of change.

Both Kaine and Bayh would help Obama in a state that Democrats are trying to put in play.