When he got into the race, many wondered if his appeal would stretch outside the northeastern liberal enclave he's represented in Congress for the past 25 years. But this summer, Sanders drew crowds in the tens of thousands in places like Portland, Ore., Los Angeles, Boston and Madison, Wis.
Then, as summer faded into the fall, some wondered whether those crowds would translate into voters. Sanders answered that definitely with his poll numbers and his very close second-place finish in the Iowa caucuses.
Where does he stand in the polls?
Sanders has steadily chipped away at Democratic front-runner Hillary Clinton's lead, both nationally and in the first two early-voting states, Iowa and New Hampshire. He closed the once double-digit polling gap enough enough to come within a razor-thin margin of beating Clinton in Iowa. Now, he leads by around 20 points in New Hampshire.
What are his strengths?
The signs are everywhere that Sanders's relentless, passionately delivered message about economic inequality and the big changes America needs make to even the playing field are sticking with potential Democratic voters.
A national November New York Times-CBS News poll showed that 56 percent of Democratic primary voters said they have a positive view of socialism.
A January Bloomberg Politics-Des Moines Register poll in Iowa showed 44 percent of likely Democratic caucus-goers there described themselves as anti-Wall Street -- a sentiment that is key to Sanders's campaign. And a June Gallup poll showed 47 percent of Americans would be willing to vote for a socialist candidate.
What are his weaknesses?
Sanders's almost-singular focus on economic inequality and other economic issues can sound out of touch in a world shocked and besieged by increasingly violent terrorist attacks and the rise of the Islamic State.
In a November Democratic debate, the day after terrorists linked to the Islamic State killed 130 people in Paris, Sanders offered his condolences before abruptly, and some would say awkwardly, pivoting to his stump speech on the dangers of billionaires.
He's since gotten more adept at talking about foreign policy, but remember: Sanders is trying to best the former secretary of state, who a Washington Post-ABC News poll in November found is the most trusted 2016 candidate -- on either side -- when it comes to terrorism.
What would it take for Sanders to win the nomination?
Sanders's main electoral struggle is appealing to minority voters, who make up a significant portion of likely Democratic voters in the two state primaries after Iowa and New Hampshire -- South Carolina and Nevada. In addition, most every other state from there on out has fewer white voters than Iowa and New Hampshire.
Sanders can reasonably claim a victory by almost winning Iowa -- he has 20 delegates to Clinton's 22. Next, he needs to earn a decisive victory in New Hampshire (where the Clinton camp will argue he essentially has home-state advantage) to have enough momentum to even come close to winning over non-white voters in the West and South. Polls consistently show Clinton way ahead in the category of non-white voters -- leading Sanders nationally by 45 points, according to a Fox News poll.
Even after defying expectations already this campaign, Sanders has a long, uphill battle to get the nomination. Still, we leave room for Sanders to surprise us again.