With the McCain-Palin team courting undecided female voters, including some who backed Senator Clinton in the Democratic primaries, Senator Obama’s aides said they were counting on not only Mrs. Clinton but also Democratic female governors to rebut Ms. Palin — and, by extension, Mr. McCain.

Senator Obama’s advisers said the candidate all but reached his decision while on vacation in Hawaii. They said the nomination process was unexpectedly intense, and was condensed because Mr. Obama did not want to start actively vetting potential running mates before Senator Hillary Rodham Clinton quit the race in June.