At Tuesday nights debate in Iowa, Democratic presidential candidates differed on the question of whether children of millionaires should get free public college.

Sens. Bernie Sanders and Elizabeth Warren have endorsed it, while Pete Buttigieg, Tom Steyer, and Sen. Amy Klobuchar explained their opposition to the idea.

Buttigieg said he supports free public college for everyone except those whose annual incomes are in the top 20%. If youre in that top quintile, he said, I just need you to go ahead and pay that tuition, because we could be using those dollars for something else.

Warren countered that her domestic policy agenda includes a wealth tax on billionaires, and that even the lowliest billionaire would be paying millions more in federal taxes each year.

If he wants to send his kid to public university, then Im OK with that, because what we really need to talk about is the bigger economic picture here, Warren said.

It may have seemed like a picayune policy squabble, but the back-and-forth was about more than education. The 2020 candidates positions on free public college illustrate larger ideological differences that are key to the future of the Democratic Party.

Democrats are debating a wide range of proposals to make life easier for families and workers. Sanders and Warren generally favor universality free health care and free public college for everybody, regardless of income. Buttigieg and the other candidates prefer limiting access to public benefits according to income. That strategy, also known as means-testing, is more in line with what has long been the Democratic Party consensus.

Over the past century, the federal government has undertaken both universal and means-tested programs. Social Security retirement insurance and Medicare are examples of universal programs, meaning just about everyone gets benefits once they reach a certain age. They also happen to be extremely popular so popular that Democrats and Republicans alike shy from any proposal that would trim benefits in order to save the government money.

Means-tested programs, on the other hand, are constantly getting kicked around. Take Aid to Families with Dependent Children, a means-tested entitlement that provided cash benefits to low-income mothers and constant grist for Republicans who said its beneficiaries were just lazy, an argument that often carried racial undertones. In the 1990s, Democrats agreed to cancel the program, hoping welfare would no longer be a political issue , but Republicans ever since have sought similar devastating reforms to other means-tested programs, such as Medicaid and food benefits.

And theres no escaping the millionaires. Last year, the Trump administration and Republicans in Congress repeatedly warned that the Supplemental Nutrition Assistance Program, commonly known as food stamps, is ripe for exploitation by the seven-figure set. Citing the single example of a wealthy Minnesota retiree who signed up for food benefits just to show that he could (because recipients are so poor that most states waive asset tests), the administration has actually set in motion a series of regulatory cuts to the program.

In short, Sanders and Warren could argue that a universal program like free college is more durable politically because it has a bigger constituency that would be mad about cuts and the political clout to push back on attempts to defund it. But neither candidate went there; Warren instead stressed the importance of sticking it to millionaires.

We need to be willing to put a wealth tax in place, to ask those giant corporations that are not paying to pay, because thats how we build an economy and, for those who want to talk about it, bring down the national debt, Warren said.

Klobuchar said that Warren is missing the big picture, and that the emphasis on postsecondary education is a mistake because the U.S. is not going to have a shortage of business school graduates.

Were going to have a shortage of plumbers, Klobuchar said. So, when we look at that, then we step back. Where should our money go? It should go into K through 12. It should go into free one- and two-year degrees, like my dad got, like my sister got.

Buttigieg has previously said the focus on four-year degrees is elitist, as Klobuchar implied. (The criticism elides the fact that Sanders and Warrens plans also cover trade schools and apprenticeships .)

Higher education is a particularly relevant topic for the public benefits debate, said Marshall Steinbaum, an assistant professor of economics at the University of Utah and senior fellow at the Jain Family Institute. Free public high schools are one of the greatest public benefits in the U.S., he said, and theres no reason political leaders shouldnt expand it to the postsecondary level contrary to claims that free public college is a giveaway to the rich.

If that seemingly populist argument is used to defeat the case for free high-quality public education available without regard to race or class status, Steinbaum said, the higher education system becomes segregated and predatory, therefore ineqalitarian and unpopulistic.