Barack Obama had begun thinking about his Senate successor even before the presidential election, and dispatched Rahm Emanuel days after the vote to contact aides of Illinois Gov. Rod Blagojevich to begin talking up Mr. Obama's preferred candidates, associates of Mr. Emanuel said this weekend. Mr. Emanuel, a congressman from Chicago, had been approached about being Mr. Obama's White House chief of staff the week before the election, though he hadn't yet officially decided to take the post. Nonetheless, the issue of Mr. Obama's Senate replacement was sensitive enough that senior Obama aides wanted to keep the matter within the...