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.

Image Democratic vice-presidential speculation has focused on three candidates, from top, Senator Evan Bayh of Indiana, Gov. Tim Kaine of Virginia and Senator Joseph R. Biden Jr. of Delaware. Credit... Photographs, from top, by Mannie Garcia/AFP — Getty Images; Alex Wong/Getty Images for “Meet the Press”; Mark Wilson/Getty Images

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.