Democratic U.S. presidential candidate Senator Bernie Sanders speaks as former Secretary of State Hillary Clinton listens at the PBS NewsHour Democratic presidential candidates debate in Milwaukee.

Hillary Clinton and Bernie Sanders battled for the crucial backing of black and Hispanic voters in Thursday night’s Democratic debate and clashed heatedly over their support for Barack Obama as the United States presidential race shifted toward states with more minority voters.

Clinton, who has cast herself as the rightful heir to Obama’s legacy, accused Sanders of diminishing the president’s record and short-changing his leadership.

“The kind of criticism I hear from Senator Sanders, I expect from Republicans. I do not expect it from someone seeking the Democratic nomination,” Clinton said in a sharp exchange at the close of the two-hour debate in Milwaukee. Her biting comments followed an interview in which Sanders suggested Obama hadn’t succeeded in closing the gap between Congress and the American people — something Obama himself has acknowledged.

Sanders responded: “Madam Secretary, that is a low blow.” And he noted that Clinton was the only one on the stage who ran against Obama in the 2008 presidential race.

Long viewed as the overwhelming front-runner in the Democratic race, Clinton has been caught off guard by Sanders’ strength, particularly his visceral connection with Americans frustrated by the current political and economic systems. Clinton’s own campaign message has looked muddled compared to his ringing call for a “political revolution,” and her connections to Wall Street have given Sanders an easy way to link her to the systems his supporters want to overhaul.

Seeking to stem Sanders’ momentum, her campaign has argued that his appeal is mostly limited to the white, liberal voters who make up the Democratic electorate in Iowa and New Hampshire. Clinton’s team says that as the race turns now to Nevada, South Carolina and other more diverse states, her support from black and Hispanic voters will help propel her to the nomination.

Attempting Thursday night to boost his own support from minorities, Sanders peppered his typically economic-focused rhetoric with calls to reform a “broken criminal justice system” that incarcerates a disproportionate number of minorities.

“At the end of my first term, we will not have more people in jail than any other country,” he said.