(AP) -- The Democratic primary field returns to the debate stage on Sunday night for the last matchup before the first round of voting next month, amid tightening poll numbers and an increasingly raucous race.

With Vermont Sen. Bernie Sanders gaining on longtime front-runner Hillary Clinton in Iowa and New Hampshire, the debate is likely to mark a sharp departure from the relatively civil exchanges that defined the last three face-offs.

Both sides are readying sharp exchanges over a series of policy issues including gun control, taxes and health care.

The debate is scheduled to take place just blocks from the Emanuel African Methodist Episcopal Church, where nine parishioners were killed during Bible study in a mass shooting last summer. Gun control has emerged as a central theme in the race, with Clinton citing the issue as one of the major differences between the candidates.

On Saturday night, Sanders announced his support for legislation that would reverse a 2005 law granting gun manufacturers legal immunity that he once supported.

His changed position came in a statement issued after days of attacks from rival Clinton, who had attempted to use his previous vote to undercut his liberal image.

Clinton immediately cast the move as a "flip-flop," in a series of interviews on Sunday morning talk shows. Sanders said he backed the 2005 law in part because of provisions that require child safety locks on guns and ban armor-piercing ammunition.

"There were things in it that I did not like, and I was willing to rethink," he said on NBC's "Meet the Press." ''We have rethought."

Clinton has also criticized Sanders' universal health care plan, arguing that his proposal would undercut President Barack Obama's signature health care law and suggesting a tax increase on the middle class might be required to pay for it.

"It would be a mistake to really thrust our country into another contentious national debate about how we're going to provide quality, affordable health care to everybody," she said on CNN's "State of the Union."

In an interview with TIME magazine on Sunday, Sanders said that his plan would ultimately save taxpayers money by lowering their health care bill.

Sanders, meanwhile, has questioned Clinton's liberal credentials, casting the former secretary of state as a Wall Street ally who will switch her position for political gain. But he's vowed to forgo negative attacks, a position that may be hard to maintain as the race continues to intensify.

Both candidates are competing for black voters in South Carolina, which hosts the fourth primary contests. At a party fundraising dinner last night, they vowed to change criminal justice policies.

Former Maryland Gov. Martin O'Malley will also be on the stage, which offers perhaps his last chance to improve his standing in the race. He's been stuck in single digits since announcing his campaign last spring.