Now that the 2014 elections are over and national politics is all about 2016, Democrats have good reason to worry that, for all his success at the polls, President Obama will leave his party with a toxic legacy.The Obama damage is two-fold. First, his success relied on a coalition that likely will not survive, or at least survive at full strength, without Obama himself on the ticket. Secondly, Obama drove a significant portion of white voters away from the Democratic Party.Put those two things together -- smaller Obama coalition and more alienated whites -- and the result could be huge trouble for whoever the Democratic presidential nominee is in 2016.First the coalition: Obama's powerful appeal to minorities, women, and young people propelled his decisive wins in 2008 and 2012.