Factbox: Four Democrats seek party's 2016 U.S. presidential nomination

(Reuters) - Former U.S. Secretary of State Hillary Clinton remains the front-runner for the 2016 Democratic presidential nomination, but she faces challenges from several current and former officials. For the latest Reuters/Ipsos poll results for all the potential Democratic presidential candidates, see http://bit.ly/1BKDRGE. Here is a list of the Democrats seeking the nomination or considering jumping into the race for the November 2016 presidential election: OFFICIALLY RUNNING HILLARY CLINTON