Mr. Obama’s aides are confident that the passions of the primary season have given way to a more pragmatic view among Mrs. Clinton’s supporters and that Mr. Obama would not risk a major backlash from women or other constituencies associated with her if the vice presidential slot goes to someone else.

But the Obama camp has done little in public to prepare Clinton supporters for the increasing likelihood that she will not be on the ticket. There is no shortage of speculation among Mrs. Clinton’s supporters that she is very much in the running as Mr. Obama begins closing in on a decision.

The list of potential candidates includes Senators Evan Bayh of Indiana and Joseph R. Biden Jr. of Delaware, as well as Gov. Tim Kaine of Virginia, and other names of varying degrees of plausibility. The Democratic convention begins Aug. 25 in Denver, and Mr. Obama was said to be considering an announcement the week before, although aides said it could be earlier.

The arguments for picking Mrs. Clinton have always been highly pragmatic, based more on electoral politics than anything else, as Mr. McAuliffe suggested in pointing to the vote-getting power she had exhibited. (For what it is worth, some of Mr. McCain’s advisers said they viewed Mrs. Clinton as the single strongest candidate Mr. Obama could pick for those same reasons.)

Those arguments have not held much sway with Mr. Obama or his inner circle. They have indicated that any political benefits gained by putting Mrs. Clinton on the ticket would be outweighed by the costs.

Mr. Obama’s advisers say that a central message of his campaign  that he would represent a break from the way politics are conducted in Washington  would be tarnished by the simple act of linking himself to the family that has dominated Democratic politics since Bill Clinton won the presidency in 1992.

In addition, a campaign marked by tight control and few displays of infighting (a structure that Mr. Obama’s advisers would like to replicate in the White House should he win) would, some of Mr. Obama’s associates said, be undermined by trying to merge with the Clintons. Several of his associates said they were particularly wary of trying to manage a campaign that included Mr. Clinton.