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Hillary Clinton and Bernie Sanders are battling more fiercely than ever as they head into their final debate before the first votes in the 2016 election campaign are cast.

With polls showing Sanders neck-and-neck with Clinton in Iowa—and ahead of her in New Hampshire—the Clinton camp has begun launching its most aggressive attacks yet on Sanders.

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The New York Times reported Saturday that many within Clinton's campaign think that they underestimated Sanders's strength, and should have taken him more seriously earlier on in the campaign.

Clinton is certainly taking Sanders seriously now. On Sunday, both Democrats went on multiple Sunday shows to trade barbs.

Clinton accused Sanders of changing his position on gun control.

Sanders accused Clinton and her surrogates of distorting his plan for universal, single-payer health care.

Both topics are sure to come up during Sunday night's debate, which is being held in Charleston, South Carolina. NBC's Lester Holt is moderating. The debate is happening mere blocks from the Emanuel AME Church, where white supremacist Dylann Roof allegedly massacred nine black parishioners. That, too, is likely to be addressed by Clinton, Sanders and Gov. Martin O'Malley.

The debate airs at 9 PM ET on NBC.