With no serious challenges for the Democratic presidential nomination in 2012, the president can target the independent voters crucial to victory, while his Republican rivals must move right to win their party's nomination.When President Obama swoops into Chicago on Air Force One to formally launch his reelection bid Thursday, his homecoming will have all the trappings of a celebrity event: high-dollar fundraisers at a pair of fancy restaurants, an adoring audience at a waterfront rally and an appearance by NBA star Derrick Rose.