The Clintons are very good at this game. They are positioning Hillary to the right of Obama, and getting her into the newscycle.
*Oooh, sorry about putting ‘positioning’ and ‘Hillary’ in proximity like that. Accident, I swear.*
Hillary is not going to challenge Obama for the presidential nomination. While she might be able to win the democrat nomination, by challenging Obama she would lose the black vote in the general election and thereby almost certainly lose to the Republican candidate. She’s blocked because Obama is an African-American. If he was a white liberal then she could challenge him and have some chance of winning the general election.