Bernie Sanders. AP Photo/Carlos Osorio Democratic presidential candidate Bernie Sanders made a quip about Republicans and mental health at the CNN primary debate on Sunday night.

During a response to a question about fund-raising, Hillary Clinton said she admired the Sanders campaign, and she took a shot at the Republican Party.

"We have our differences," she said of Sanders. "And we get into vigorous debate about issues. But compare the substance of this debate with what you saw on the Republican stage last week."

The crowd cheered loudly.

Sanders used his next question to go back and elaborate on Clinton's point.

"We are, if elected president, going to invest a lot of money into mental health," the Vermont senator said. "And when you watch these Republican debates, you know why we need to address the mental health."

Clinton laughed, and Sanders continued his answer about the campaign-finance system.