1. Senator Obama is sliding in the polls. 2. Most of the undecided voters are supporters of Senator Clinton who haven't committed to a candidate yet. 3. Picking Senator Clinton would be a huge "Wow!" factor. 4. Senator Obama may not want her but practical political considerations(namely, winning the election) may carry the day. Also, since most of the undecided voters are pro-choice that may be why Senator McCain is considering choosing Joe Lieberman or Tom Ridge.