Hillary Clinton and Bernie Sanders go head-to-head Sunday in a CNN Democratic debate certain to focus on racial barriers and economic fairness but that will above all be defined by the toxic water crisis afflicting its host city, Flint, Michigan.

The former secretary of state and the Vermont senator, who are locked in a monthslong duel for the Democratic nomination, will use the debate starting at 8 p.m. ET to court minority and working-class voters ahead of Michigan’s primary on Tuesday.

Their showdown comes a day after Sanders pulled off morale-boostingSuper Saturday victories in caucuses in Kansas and Nebraska, doubling up on Clinton, who won one matchup — the Louisiana primary.

Yet such is the math of the presidential nominating process that Clinton won 57 pledged delegates to the 50 hauled in by Sanders, and so extended her lead in the overall Democratic presidential race.

The rivals also face off Sunday in Democratic caucuses in Maine, where polls will close just as they step onto the debate stage.

The latest election contests underlined one of the structural challenges for Sanders as he tries to enact a “political revolution” by halting the Clinton political machine.

Sanders fares well in less racially diverse states, including those he won Saturday. Clinton does better among states with high proportions of African-American voters — which gives her a more credible road to the Democratic nomination.

“(The) truth is we have not done well in the Deep South,” Sanders told CNN’s Dana Bash on “State of the Union” on Sunday.

“But I am absolutely certain that our numbers will continue to get better,” Sanders said, touting his seven primary and caucus wins, which he said showcased his advantage over Clinton among younger voters.

The Democratic presidential campaign is unfolding amid themes of race, economic opportunity, and the problems afflicting blue-collar workers in an age of globalization that are crystallized in the city of Flint.

The water crisis erupted there when government officials switched the city’s water source temporarily in April 2014 from the Great Lakes to the Flint River to cut costs. Pollution from the highly corrosive river water then ate into the city’s water system, causing lead to leach into the water supply.

Residents say that even though the supply has now been switched back to Lake Huron, the damage done to piping in their homes and in the water infrastructure remains severe and they are forced to drink bottled water. Michigan officials are investigating the extent to which lead poisoning has harmed the residents, including children, amid reports of severe rashes, developmental issues and other health problems consistent with lead poisoning.

Democratic presidential candidates have seized on the situation as proof of the indifference of government toward ordinary people, and have scorched Michigan’s Republican Gov. Rick Snyder, who has apologized for the actions of state officials.

The former secretary of state says that had the water crisis been discovered in a mainly white area, it would have been fixed long ago.

“This is not merely unacceptable or wrong, though it is both. What happened in Flint is immoral,” Clinton said during a visit to the city last month.

Clinton’s focus on the plight of Flint came at a time when she was seeking to cement her support among African-American voters in states including South Carolina, Georgia and Alabama, to combat the surprising strength of the Sanders campaign.

Sanders visited Flint last month to meet residents and to hear horror stories about their contaminated water supply, and later at a rally in Dearborn expressed outrage over the crisis.

“The bottom line is, when you left that meeting, you thought, ‘What country am I living in? Is this the United States of America?’ I fear that Flint is the canary in the coal mine here,” he told the crowd.

But Flint’s problems are not limited to its water crisis alone. The city has long been a symbol of economic blight and the human implications of the decline of large-scale industrial manufacturing following the loss of its GM car manufacturing plant. More than 41% of Flint’s residents live below the poverty level, according to the U.S. Census.

Sanders is certain to press home his dominant campaign message that the U.S. economy is run by billionaires and stacked against the working and middle classes and to argue that Clinton is far too close to Wall Street.

“(The) middle class in this country is disappearing, we have massive income and wealth inequality,” Sanders said in his CNN interview, hitting Clinton over her past support for free trade. He will also likely again challenge his rival on Sunday night to release transcripts of paid speeches she gave to mighty financial firms.

But Clinton argues that Sanders proposals on issues such as free college and a state-run health care system have little chance of being enacted in Washington.

“Anyone running for president owes it to you to come up with real ideas. … a credible strategy designed for the world we live in now,” Clinton said this week, implying that the ideas Sanders preaches would be impossible to put into practice.

As reported by CNN