Washington - Senator Barack Obama has all but settled on his choice for a running mate and set an elaborate rollout plan for his decision, beginning with an early morning alert to supporters, perhaps as soon as Wednesday morning, aides said.

    Mr. 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 Mr. Obama issued to staff members: that his decision not leak out until supporters are notified.

    Mr. 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, Mr. Obama was said to be focused mainly on three candidates: Senator Evan Bayh of Indiana, Gov. Tim Kaine of Virginia and Senator Joseph R. Biden Jr. of Delaware.

    Some Democrats said they still hoped that he would choose Senator Hillary Rodham Clinton of New York, or Gov. Kathleen Sebelius of Kansas, who has been under steady consideration by Mr. Obama's campaign.

    By all indications, Mr. Obama is likely to choose someone relatively safe and avoid taking a chance with a game-changing selection. A similar strategic choice now faces Mr. Obama's Republican rival, Senator John McCain of Arizona, who has been under pressure from some Republicans to make a more daring choice.

    Mr. Obama's advisers said he all but reached his decision while on vacation in Hawaii. They said it was 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 Mrs. Clinton quit the race in June.

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

    That gave Mr. Obama's team of lawyers less time to review candidates, and several Democrats said it appeared that the list of candidates who were deeply vetted was limited to about a half-dozen people. (Campaigns typically check the background of candidates who are not necessarily in play, as a way of gaining favor with various constituencies or to keep the other party off balance.)

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

    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.

    Mr. Obama and his running mate will begin, perhaps that day, a visit to swing states. Plans call for them to be on the trail together for much of the time between the day of the announcement and the day Mr. Obama arrives in Denver, a week from Wednesday, but their most intense campaigning together will come after the convention.

    Mr. Obama's schedule calls for him to awaken on Tuesday in Orlando, Fla., and by the end of the day be in Raleigh, N.C. By Wednesday, he is scheduled to be in Virginia. The Obama campaign has cautioned against reading anything into his schedule, saying it could be changed in an instant to accommodate the plan to introduce the running mate.

    Aides said the announcement would come at the earliest on Wednesday morning.

    Mr. Obama's advisers said they wanted to time the announcement to get maximum publicity going into the convention, after a stretch in which Mr. Obama was on vacation in Hawaii and Mr. McCain made good use of having the political stage largely to himself. Vice-presidential announcements are one of a handful of moments when the presidential candidates are given a clear grab at the public spotlight, and both Mr. Obama and Mr. McCain have put much thought into the timing of their announcements.

    If Mr. Obama is looking to build excitement going into the convention, Mr. McCain's aides have looked to announcing his choice right after the Democratic convention, which ends Aug. 28, a Thursday, as a way of stepping on whatever bounce Mr. Obama enjoys from his nomination.

    The Republican convention begins the following Monday.

    Democrats close to the process said the ability to turn up information on the Web had made it easier for Mr. Obama's search team - Caroline Kennedy and Eric Holder, a former deputy attorney general - to plumb the backgrounds of prospective running mates with relatively little notice. In addition, because so many of the candidates were senators, they were required to file annual financial disclosure reports.

    Mr. Holder and Ms. Kennedy have been working largely out of Mr. Holder's law firm in Washington, using lawyers in his firm and others - many of whom are veterans of the process from having worked for Senator John Kerry in 2004 and Al Gore in 2000 - to check the backgrounds of the potential candidates.

    Mr. Obama was briefed frequently throughout the proceedings - receiving updates by telephone and e-mail - and came to Washington for a handful of meetings with a small group of senior advisers in the law offices of Covington & Burling, where Mr. Holder is a partner. With the vetting concluded, there was no activity on Monday in the firm's suite of offices on Pennsylvania Avenue.

    Unlike in previous selections, Mr. Obama has been largely spared the obligation of staging elaborate cloak-and-dagger processes to interview prospective running mates because he has been campaigning with them in close quarters, giving him a chance to get to know them.

    The rampant speculation during the selection process encompassed many of the best-known names in the party, including Mr. Kerry and Mr. Gore, Senator Christopher J. Dodd of Connecticut, Senator Jack Reed of Rhode Island, Gov. Bill Richardson of New Mexico and former Senator Sam Nunn of Georgia.

    The big choice for Mr. 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 who might help him win a contested state.

    Mr. Biden, the chairman of the Senate Foreign Relations Committee, clearly fell into the category of helping Mr. Obama on foreign policy, while Mr. Kaine is a relative newcomer to national politics and would reinforce the notion of change. Both Mr. Kaine and Mr. Bayh would help Mr. Obama in a state that Democrats are trying to put in play.

    For all the attention to Mr. Obama's deliberations, it is by no means assured that his choice will make a big difference in the outcome of the campaign.

    "Vice-presidential candidates can make a marginal difference," said Matt Bennett, the co-director of Third Way, a Democratic advocacy group, "but they rarely matter in terms of winning a state or region - as Mike Dukakis and John Kerry found out. And a weak candidate doesn't really drag the ticket into the drink - as George H. W. Bush found out."