The 74-year-old Vermont senator overcame a 40-percentage-point deficit when he entered the race in spring 2015 to in February beat Hillary Clinton in the first-in-the-nation New Hampshire primary.
In doing so, he became the first Jewish candidate and first self-described "democratic socialist" to win a major-party presidential nominating contest.
To get where he has, Sanders has hardly broken from a narrow message that blamed Wall Street and unchecked campaign donations for much of the nation's woes. He also has done little to change his image of unkempt white hair and rumpled suits, despite being a disciplined veteran politician.
From '60s activist to Vermont institution
Sanders has been working on liberal causes for almost six decades, starting from his time as a student activist at the University of Chicago, where he worked to desegregate student housing as a member of the Congress of Racial Equality. As a college student, he also participated in the March on Washington led by the Rev. Martin Luther King Jr.
Four years after graduating college, Sanders moved with his first wife to Vermont as part of a migration of liberal activists to the state. He later joined the fledgling Liberty Union Party in 1971 and volunteered to run for the U.S. Senate, beginning a string of crushing statewide losses that lasted through the '70s.
During his decadelong losing streak, Sanders started a company making educational films and narrated a video about Eugene Debs, the last major socialist candidate to seek the White House, roughly a century ago.
It was not until he left the Liberty Union Party that Sanders found success, running as a socialist for mayor of Burlington, Vermont. He spent the next decade leading the biggest city in Vermont and developing his name as something of an oddity in American politics: a successful socialist.
In 1990 he ran for Congress and began a career in Washington, spending 16 years in the House of Representatives and close to a decade in the Senate.
Sanders hasn't traditionally talked much about his family on the campaign trail, but he slowly began referring to them increasingly at the urging of his second wife, Jane Sanders.
Sanders was born in Brooklyn in 1941 to a father who left Poland to seek work in the United States and a mother who was the daughter of Polish immigrants. Despite his potential to become the first Jewish president, Sanders has been circumspect about his feelings on organized religion and instead takes a view that all humans are connected.
His religious views sound very much like his political views.
"I believe that, as a human being, the pain that one person feels, if we have children who are hungry in America, if we have elderly people who can't afford their prescription drugs, you know what, that impacts you, that impacts me," Sanders told CNN's Anderson Cooper at a Democratic town hall in New Hampshire a week before the primary.
"So my spirituality is that we are all in this together and that when children go hungry, when veterans sleep out on the street, it impacts me," he said. "That's my very strong spiritual feeling."
Sanders married his first wife and then moved to Vermont after college. His first son, Levi, was born in 1969, after Sanders' first marriage ended. Sanders married his second wife, Jane O'Meara, in 1988. She was a fan who saw him debate during his Burlington mayoral race in 1981. According to a People magazine article, he proposed to her in the parking lot of a Friendly's.
Clinton entered the Democratic contest as the prohibitive favorite last spring, easily outpacing all serious possible contenders, with the exception of Vice President Joe Biden, had he entered the race.
The least likely challenger seemed to be an aging socialist activist from Vermont.
In reality, however, Sanders spent much of 2014 steadily laying the groundwork for a serious White House bid: meeting with liberal leaders in Washington and headlining Democratic events in Iowa and New Hampshire.
He formally announced his bid for the White House on a small patch of the Capitol lawn in April 2015 attended by a handful of reporters, although a month later he held a more spirited kickoff in his hometown of Burlington.
Over the summer he drew large crowds to rallies, driven by an almost rock star-like fervor among fans who online said they could #FeelTheBern. By September, Sanders had overtaken Clinton in most New Hampshire polling and was closing a double-digit gap in Iowa.
Comedian Larry David's dead-on impersonation of Sanders on "Saturday Night Live" in the fall led to some unexpected celebrity for Sanders and was followed up in February with David playing Sanders on SNL in "Bern Your Enthusiasm."
Meanwhile, the Vermont senator's amplified digital fundraising operation, led by veterans of President Barack Obama's campaigns, drew in surprising cash hauls -- more than enough to keep pace with Clinton. By February, Sanders announced he had out-raised Clinton in the previous month by $5 million.
But Sanders has also faced a deficit of support among minority voters. He also staffed up much later than Clinton, forcing him to play catch-up in other early nominating states.