WASHINGTON -- President Barack Obama's top campaign advisers said Tuesday they are uncertain about which Republican challenger will emerge next year but are predicting a long GOP primary contest that they say will produce a weaker opponent in 2012.Democrats have targeted former Massachusetts Gov. Mitt Romney as the most likely Republican challenger to Obama in recent weeks but now say former House Speaker Newt Gingrich's rise in the polls has made the GOP contest very unpredictable.Obama campaign strategist David Axelrod said in a briefing for reporters that he was unsure "what kind of candidate will be in the general election." He said anticipated a lengthy primary contest that would eventually hurt the party's nominee.Read More... More on Barack Obama