Bernie Sanders is running for the Democratic nomination even though he has never officially described himself as a Democrat before. A self-described Democratic Socialist, the Vermont senator is an independent in politics and chooses to caucus with the Democrats in the Senate. Sanders has had a long career in politics, rising from mayor of Burlington (1981 to 1989) to the U.S. House (1991-2007), then the Senate (2007-present).
When his campaign started, Sanders was viewed as a kind of protest candidate. The unofficial but widely-acknowledged leader of the liberal wing of the Democratic Party, Massachusetts Sen. Elizabeth Warren, decided not to run despite an effort to draft her into the race. So Sanders opted to run instead to represent similar views.
As expected, Sanders has proposed policies, like free tuition at all public colleges and a Medicare-for-all health system, that are to the left of Hillary Clinton and the Democratic establishment.
A big part of Sanders' authenticity is his consistent commitment to economic inequality reform. He bemoans rising income inequality and the growing concentration of wealth. His message has helped close the gap nationally between him and Clinton, but is viewed as unlikely to defeat Clinton.
So far in the primary contest, Sanders has seen victories in New Hampshire, Vermont, Colorado, Oklahoma, and Minnesota.