Democratic presidential candidate Hillary Clinton responds to opponent Bernie Sanders's suggestion that the United States can learn from countries like Denmark. (CNN)

Raise your hand if you had "Hillary Clinton defends capitalism, and/or criticizes Denmark" in your office pool for the first on-stage debate fight of the Democratic primaries. That's right - you didn't. But Clinton's extolling of the free-market economic system, and her critique of Democratic socialism, was her first open attack on the man closest to her in the polls, Sen. Bernie Sanders of Vermont. It showed an important fault line in this primary campaign.

The debate opened with the moderator, CNN's Anderson Cooper, smacking each of the five candidates on stage with her or his biggest perceived vulnerability in this race. For Sanders, it was his long and proud identification as a socialist, a term that, Cooper said, half of Americans find disqualifying in a candidate for president. "How can any kind of socialist win a general election in the United States?" Cooper asked.

Sanders responded with an attack on what he called "casino capitalism," an economic system that largely rewards the very rich and leaves the poor and middle class behind. He said the United States should be more like Scandinavian countries, which provide larger safety nets for their populations.

"When you look around the world, you see every other major country providing health care to all people as a right, except the United States," Sanders said. "You see every other major country saying to moms that, when you have a baby, we're not gonna separate you from your newborn baby, because we are going to have -- we are gonna have medical and family paid leave, like every other country on Earth.

"Those are some of the principles that I believe in, and I think we should look to countries like Denmark, like Sweden and Norway, and learn from what they have accomplished for their working people."

Clinton fired back with a defense of capitalism as a concept. First she redefined it - capitalism, she said, is about entrepreneurs being able to start small businesses - and then she said America must save it from itself. "We are not Denmark," she told Sanders. "I love Denmark."

"We would be making a grave mistake to turn our backs on what built the greatest middle class in the history of the world," she said.

That this is a real debate in a major political party in the United States reflects several changes in the economy, and one big shift among Democratic voters.

Hillary Clinton and Bernie Sanders and the other candidates took on gun control, Benghazi and other big issues at the first Democratic presidential debate of the 2016 race. Here are the highlights. (Sarah Parnass/The Washington Post)

Scarred by the Great Recession and 25 years of middle-class wage stagnation, a majority of Americans now see the economy as stacked against people like them. They're still angry at Wall Street after the financial crisis, and, particularly on issues such as family leave, many of them look fondly upon policies like Denmark's.

These trends have continued under the recovery overseen by a Democratic president, Barack Obama. As they have, Democratic politicians have elevated inequality and middle-class stagnation to the top of their economic agendas. There's a real debate on the role of the free market in addressing those issues. It comes down to the question of whether capitalism needs to be fixed up, or overhauled completely. The difference between fixing and overhauling is the main policy difference between Clinton and Sanders, as the debate showed.