Charleston, S.C. - The Democratic primary field returns to the debate stage on Sunday night for the last matchup before the first round of voting next month.

Vermont Sen. Bernie Sanders is gaining on longtime front-runner Hillary Clinton in Iowa and New Hampshire, and 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 in a mass shooting last summer. Gun control has emerged as a central theme in the race. Clinton points to the issue as a major difference between the candidates.