Hillary Clinton and Bernie Sanders have been relatively civil so far during their months-long contest. But tensions in the Democratic presidential campaign are poised to flare in Charleston, South Carolina, Sunday night over curbing gun violence.
With two weeks before the Iowa caucuses, Clinton, Sanders and former Maryland Gov. Martin O'Malley were on a debate stage just a few blocks from Emanuel African Methodist Episcopal Church, where nine parishioners were killed during Bible study in a mass shooting over the summer.
Clinton has used Sanders' vote on a 2005 law giving gun manufacturers legal immunity to undercut his liberal image. Sanders announced Saturday, on the eve of the debate, that he was reversing his position and is now supporting legislation that would amend the liability law.
Clinton's once formidable lead in Iowa has dwindled, and the Vermont senator has maintained a steady advantage in neighboring New Hampshire, adding a sense of urgency in the last debate before the Feb. 1 Iowa contest and the New Hampshire primary a week later.
Clinton entered the 2016 race as the prohibitive favorite for the nomination, and she has spent much of her time tangling from afar with Republicans, arguing she is best candidate to build upon President Barack Obama's agenda. But Sanders has become a more immediate threat.
Sanders has a passionate following among young voters and liberals. For months, he has pointed out his differences with the former secretary of state, most notably his plan to break up large Wall Street banks.
His latest TV ad suggests he would be tougher than Clinton on Wall Street. That's led Clinton's team to say he crossed the line he pledged not to cross into negative campaigning.
Some things to watch at the debate sponsored by NBC News and YouTube at 9 p.m. ET.
CLINTON THE AGGRESSOR
In the days before the debate, Clinton has railed against Sanders' support for the 2005 law that gave immunity to gun manufacturers, saying he was unwilling to stand up to the National Rifle Association. Clinton praised Sanders' reversal on Sunday, telling CNN's "State of the Union" she was "very pleased that he flip-flopped on the immunity legislation."
She also says his universal health care plan would undercut Obama's signature health care law and suggests it might require a tax increase on the middle class to pay for it.
Clinton was likely to take a similar tone in the debate, pointing to her experience and toughness as essential to take on the Republicans.
But such aggression carried risks for Clinton, who will need Sanders' enthusiastic supporters next fall if she wins the nomination. If Clinton repeatedly were to attack Sanders, the "Feel the Bern" crowd might not forget it.
DOES BERNIE GO NEGATIVE?
Sanders faces a similar quandary. He has bemoaned negative campaigning and during the first debate declined to rebuke Clinton over her use of a private email server while at the State Department, saying that people were "sick and tired about hearing about your damn emails."
Clinton's campaign says his Wall Street ad crossed the line, a charge Sanders' team has dismissed.
Part of Sanders' appeal is his unconventional background and his authenticity on issues such as income inequality. He's a 74-year-old Brooklyn native who describes himself as a democratic socialist.
If Sanders were to get into a nasty back-and-forth with Clinton, he might look like the sort of Washington politician he and his supporters condemn.
The debate setting lends poignancy to the political dog fight. And Sanders' election-eve reversal on gun policy adds intrigue.
Clinton has criticized Sanders on the liability vote and frequently notes that he opposed the 1993 Brady bill, which required background checks on gun purchases from federally licensed dealers, highlighting a rare area where the Vermont senator is at odds with liberals.
Sanders on Saturday said he would support legislation that would reverse the 2005 law granting gun manufacturers legal immunity. He said he wants the bill to include an amendment that would require the federal government to monitor and report on the law's effect in rural areas.
In the past, he has supported immunity for gun manufacturers by saying he wanted to protect small stores in his home state.
His campaign aides said the decision is not a flip-flop. Sanders backed the 2005 law in part because of provisions that require child safety locks on guns and ban armor-piercing ammunition. And his advisers have said that Clinton has taken multiple positions on gun control during her career.
The debate will happen a few blocks from Emanuel African Methodist Episcopal Church, where a 21-year-old white man shot and killed nine people during Bible study last June.
Clinton is likely to go after Sanders on background checks. She has said she would close a loophole under which the government only has three days to complete the check before a buyer can buy a gun. The accused Charleston shooter, Dylann Roof, was able to buy the gun used in the shooting in part because of the provision.
Sanders would not commit to closing the so-called Charleston loophole, saying on several Sunday talk shows, saying on ABC's "This Week" that "we're going to take a look at that as well. But the issue here is what my view has always been, and what is most important, is that we have a strong instant background check."
Clinton has aligned herself with Obama's push for tighter restrictions on firearms, while Sanders has backed the president's recent executive actions to expand background checks on certain gun purchases.
TAKING OVER FOR OBAMA
The two-term president is popular among Democrats, and Clinton, his former secretary of state, will try to present herself as his rightful heir.
Clinton tells audiences that the president doesn't get the credit he deserves for rescuing the economy. She promises to build upon his health care law, take up his push to overhaul immigration laws and curb gun violence.
Embracing Obama may be another way for Clinton to drive a wedge between Sanders and Democratic voters.
In an interview with MSNBC, Clinton said Sanders' Wall Street ad was a "very direct criticism of President Obama," noting that Wall Street banks were among the president's most generous campaign contributors in 2008. "That didn't stop him from fighting for the hardest regulations on Wall Street since the Great Depression," she said.
O'MALLEY'S LAST STAND
The former Maryland governor barely qualified to appear on debate stage and has been unable to break out of single digits in preference polling.
He is running low on money, cannot afford a big television advertising buy and is banking on a better-than-expected showing in Iowa's caucuses. But he is running out of time.
The debate may be the chance for O'Malley to connect with undecided caucus-goers looking for a fresh face to lead the party.
His most recent debate performance and appearances at recent forums have been well-received by Democrats. But he cannot afford to be a quiet bystander as Clinton and Sanders duke it out.