Sunday's NBC News-YouTube debate comes as Hillary Clinton and Bernie Sanders escalate their attacks on one another over issues like health care and gun control. The two leading candidates will be joined by former Maryland Gov. Martin O'Malley. So what are the big issues dividing Democrats? Here's a quick primer to get you ready for the first Democratic debate of 2016.
Clinton has sharpened her attack against Sanders' health care plan, claiming Thursday night that the senator wants to "end all the kinds of healthcare we know" on MSNBC's Rachel Maddow. While Sanders has never said outright that he would "repeal and replace" the Affordable Care Act—a common Republican sound bite—he supports implementing a single-payer system, which is different than the current health care law.
Sanders voted for the Affordable Care Act, but notably called it a "good Republican, Romney type-program" that only "addresses some needs" in 2013. The Vermont senator has continually touted the need to expand Medicare and health insurance to all Americans, including undocumented immigrants.
"I want to end the international embarrassment of the United States of America being the only major country on earth that doesn't guarantee health care to all people as a right, not a privilege," he said in the November Democratic debate.
Clinton has come down hard on Sanders for failing to disclose the details of his "Medicare-for-all" single-payer plan.
"There is no way that can be paid for without raising taxes on the middle class. The arithmetic just doesn't add up," Clinton said Tuesday in Iowa. "I don't think that is the right way to go."
Sanders does not deny a tax hike, saying on MSNBC on Wednesday that he would levy new tax fees to fund it. The campaign has promised that they will roll out full details in the coming weeks.
However that did not stop Chelsea Clinton's for stirring the pot while campaigning for her mom in New Hampshire.
"Sen. Sanders wants to dismantle Obamacare," Chelsea Clinton said at a campaign stop earlier this month.
The Sanders campaign quickly rebuked the statement and even sent reporters a 1993 thank you note from Hillary Clinton to Sanders for his "commitment to real health care access."
Politifact ranked Chelsea Clinton's claim "mostly false."
Clinton, who has pitched herself as the protector of the Affordable Care Act, would also take steps to reform the law.
According to her health care plan, Clinton wants to lower the price of drug costs by capping expenses at $250 a month (about $3,000 per year). Sanders hopes to lower costs by having Medicare negotiate prices with drug companies and determining a way to import prescriptions from Canada.
All candidates, including O'Malley, support repealing the "Cadillac tax," an excised tax on high-cost health insurance plans.
Clinton, Sanders, and O'Malley are all pushing for core Democratic principles on gun reform like expanding background checks and closing vendor loopholes. So for the Democrats, the differences on reform lie more on their records than their policy positions.
Sanders in particular has come under fire by Democrats on his votes against expanding background checks and allowing gun victims to sue manufacturing companies. His votes have led the Brady Campaign for Gun Prevention and mass shooting victim former Rep. Gabby Giffords to champion Clinton as the candidate with a "proven record" on gun reform.
Though O'Malley has not proposed a detailed plan on gun reform, he often touts proposals he was able to pass as Maryland governor as an outline. Following the Newtown school shooting, O'Malley signed into legislation a law banning 45 types of assault weapons, limiting magazine clips to ten bullets and requiring fingerprint registration at the time of purchase.
Sanders has used his populist message to separate himself from the former New York senator when it comes to Wall Street. This week, Sanders released his toughest ad yet on Clinton and Wall Street, though he did not mention his rival by name.
"There are two Democratic visions for regulating Wall Street. One says its ok to take millions from big banks and then tell them what to do," Sanders says in the ad, clearly hitting Clinton's previous record on Wall Street.
Clinton's campaign pushed back on the ad, citing that Sanders had broken his promise to run a negative-ad-free campaign.
On policy, Clinton is against reinstating the Glass-Steagall Act—which separated investment-banking and commercial-banking activity—as a cure-all. Sanders supports reinstating the act and breaking up big banks as a way to effectively regulate Wall Street.
Clinton, Sanders, and O'Malley have different visions for recent proposals put out by the president on college affordability. The president, who has asked for making community college free, has found support with Sanders, while Clinton and O'Malley are not completely on board.
Clinton supports free community college and lowering college costs. However, she is against high income families from sending their children to college for free.
"I disagree with free college for everybody. I don't think taxpayers should be paying to send Donald Trump's kids to college," she said at a November Democratic debate.
Instead, Clinton is pushing for a "compact," where students would work to help pay for their education. Her plan also proposes making applying to federal aid easier, a plan all Democratic candidates endorse.
Sanders proposes making all public colleges and universities tuition-free, a move made possible by taxing a fraction of a percent on Wall Street trades and having high income earners pay more in taxes.
O'Malley's campaign focuses on making college-debt free at in-state public colleges and universities. A President O'Malley would tie tuition rates to 10% of state median incomes in order to regulate debt.
Democrats waited and waited for Clinton to take a position on the Trans-Pacific Partnership trade agreement. And when she finally came out against TPP last October, Sanders said welcome to the club, despite her earlier support for the comprehensive trade agreement.
As secretary of state, Clinton described TPP as "the gold standard in trade agreements." But in an interview with PBS News Hour in October, Clinton no longer stood by the plan arguing that it would hurt job growth and would not regulate currency manipulation.
"What I know about it as of today, I am not in favor of what I have learned about it… I don't believe it will meet the high bar I have set," she said.
Sanders has voted against recent trade deals, including NAFTA and CAFTA, citing that they have shipped jobs overseas.
"I'm not saying trade is the only reason, but it is a significant reason why Americans are working longer hours for low wages and why we are seeing our jobs go to China and other low-wage countries," he said last year on CBS' Face the Nation.
TPP, he argued, would benefit Wall Street at the cost of American jobs.
This article first appeared on NBCNews.com.