The Democratic presidential debate Sunday night offers the Vermont senator one of his best chances to make the case to Democrats that he would be a stronger nominee than Hillary Clinton, who until recently took a hands-off approach to her chief primary rival.
The face-off at 9 p.m. ET on NBC is the final scheduled meeting before the February 1 Iowa caucuses and comes as Sanders is surging in some early-state polls.
But he faces multiple challenges as he prepares to square off against Clinton.
For one, she is often at her best when her back is against a wall -- as she demonstrated last fall during an 11-hour grilling by House Republicans over Benghazi or after losing early contests to then-Sen. Barack Obama in 2008.
Meanwhile, the debate's location -- just a block from the site of last year's racially motivated church shooting -- and timing on the heels of the Iran nuclear deal's implementation could give Clinton an advantage in gun control or foreign policy discussions. Those are areas where Sanders has struggled.
Sanders got a taste of the challenge that would await him past Iowa and New Hampshire on Saturday night, as he, Clinton and former Maryland Gov. Martin O'Malley took turns speaking at a South Carolina Democratic Party dinner and the famous fish fry hosted by Rep. Jim Clyburn.
Without mentioning Sanders' name, Clinton landed some thinly-veiled blows on gun control, reminded the largely African-American crowd that it is "one of our first lines of defense" in the Democratic primary and latched herself to President Barack Obama, saying she "thought I was going to fall on the floor" when he sang "Amazing Grace" in the wake of the Charleston church shooting.
"I have noticed that very often my name is linked to the president," Clinton said at the dinner. "Now I personally consider that a great compliment."
Clinton has reason to come out swinging: Polls this week have shown her trailing in New Hampshire, and either behind or within the margin of error in Iowa. It now looks like she could lose the first two states in the nominating contest -- and with them, momentum that her supporters once hoped would allow for a quick and decisive victory.
Clinton dominates Sanders nationally, beating him by 25 percentage points in an NBC/Wall Street Journal poll released Sunday
. And though they both beat GOP front-runner Donald Trump in a hypothetical general election matchup, Sanders has a stronger advantage in the poll.
Where things went wrong
Clinton's friends and allies are pondering where her campaign went wrong.
The most common answer lies in Clinton's decision to avoid even saying Sanders' name for months. Clinton instead trained her fire on Republicans, hoping to show her party's base that she is best prepared to compete in a general election. In that time, without an opponent's scrutiny, Sanders was able to grow from a long-shot single-issue candidate into the leader of a movement that inspired young people and liberals that Clinton hasn't quite reached.
Clinton's strategy of ignoring Sanders was clear even in the last Democratic debate in December, where she took several swipes at her rival over gun control and health care but mostly focused her attacks on Trump.
Former President Bill Clinton himself is among the Democrats critical of the campaign's handling of Sanders, The New York Times reported
Clinton's decision not to push for more Democratic debates, or to shift them from weekends when audiences would be smaller, also could have hurt her cause, since the former secretary of state has excelled on stage so far.
There's another, potentially harder-to-solve element: Clinton's brand of policy prescriptions, toughness and problem-solving could be out of sync with more deeply dissatisfied liberals who have yearned for Sanders' fire-and-brimstone condemnations of the nation's power structures -- and there's little Clinton can do to change it.
Sanders was selling his long history as a civil rights activist at the South Carolina Democratic dinner Saturday night, recalling his attendance at the Rev. Martin Luther King Jr.'s March on Washington in 1963.
As he hammered Republicans for obstructing voting rights, Sanders said of GOP governors and legislators: "If you don't have the guts to participate in a free and fair election, get another job."
Challenges for Sanders
Still, South Carolina is a microcosm of the challenge Sanders faces in expanding his base beyond the whiter, liberal electorates of Iowa and New Hampshire, the site of the nation's first primary on February 9. If he is to present a long-term threat to Clinton, he must win over a broader coalition of voters who make up the party's base.
That the race is this competitive, however, has surprised Sanders and his aides.
In October 2015, Sanders barely had any staff in South Carolina. He was focused on Iowa, because it votes first, and New Hampshire, because he has a home-field advantage as the senator from a neighboring state.
Since then, though, Sanders' campaign has ramped up spending in South Carolina and other early-voting states such as Nevada, including hiring staff, opening offices and getting on air with radio and television ads.
This has all come at a cost: Sanders spent about 90% of the money he raised in the last three months of 2015. But his aides see value in pumping as much money into the race as possible right now in a bid to seize on his growing momentum.
Clinton faces a delicate balance, too, of distinguishing herself from Sanders without alienating Democrats whose support she will need if she ultimately wins the nomination.
For months, her campaign has publicly predicted a long slog through the Democratic primary. But, as a spate of ever-more-dire fundraising pitches to supporters have shown in recent days, Clinton's allies haven't responded with a sense of urgency.
Campaign chairman John Podesta wrote on Saturday evening to supporters: "We always knew this race would get tight, and that's what's happening now. Public polling shows we have a real race on our hands in Iowa and New Hampshire -- and make no mistake, we could lose one or both of these contests."
He added: "I'm not an alarmist; I'm a realist."