WASHINGTON (AP) — Pete Buttigieg's latest ad on college affordability was a relatively quiet one: The presidential candidate is seen explaining his plan for free public college tuition for some to a small group of nodding middle-aged voters, his measured tone hardly shifting as he takes an indirect swipe at his Democratic rivals.

But the message was received and returned — in all caps.

"Universal public systems are designed to benefit EVERYBODY!" Rep. Alexandria Ocasio-Cortez tweeted in defense of Sen. Bernie Sanders' college plan, using her social media muscle to unleash a fresh barrage of tweets, posts and debate about how best to overhaul the way Americans pay for higher education.

The heated exchange exposed the potency of one of the sleeper issues of the Democratic presidential primary. College affordability may not get the attention of "Medicare for All" or carry the emotional punch of debates over race and gender, but it stands as one of the sharpest policy divides between the leading candidates in the race and one likely to have staying power.

As their party's electoral fortunes increasingly depend on college graduates, Democrats are under pressure to do something about Americans' mountain of student debt — a $1.5 trillion behemoth. Their search for solutions is creating conflicts about how to best address inequality, but the debate is also about how to best motivate college graduates to vote Democratic during the general election next year against President Donald Trump.

"There are a lot of dividing lines in politics right now — but the diploma divide is a real cleaver among voters between those who are open to Trump and adamantly opposed to Trump," said Jesse Ferguson, a Democratic party strategist.

The plans up for debate fall into familiar camps — those who want incremental changes, such as Buttigieg, former Vice President Joe Biden and Minnesota Sen. Amy Klobuchar, and those who call for a creation of a large-scale new government benefit, such as Massachusetts Sen. Elizabeth Warren and Sanders. Those favoring more incremental change are sold as fair and pragmatics, given the general opposition that Republicans and older voters have toward free college tuition.

"I only want to make promises that we can keep," Buttigieg says in his ad, which is running in Iowa, home to three major public universities in communities where younger people are increasingly clustered. Buttigieg's proposal would offer free public college tuition for students from households earning less than $100,000 a year.

"What I'm proposing is plenty bold," he said. "These are big ideas. We can gather the majority to drive those big ideas through without turning off half the country before we even get into office."

Roughly 70% of U.S. households would qualify for free tuition under Buttigieg's plan, which does not appear to make allowances for families who live in places with a high cost of living, such as parts of New York and California.

Buttigieg's plan is a step bolder than what's offered by Biden and Klobuchar, both of whom call for free community college and increasing federal financial aid.

On the opposite end of the spectrum are Warren and Sanders, who propose access to free public college tuition for all while offering to cancel much — if not all — of the existing student debt.

The plans are rooted in the idea that a universal plan optimizes the benefits to society and secures the broadest political support. Advocates often point to Social Security — the most politically durable of all entitlement programs — as a model.

"The promise of universal free access can cut through yearly budgetary fights, reduce bureaucratic hurdles to access, and increase citizens' trust in and willingness to use the program," said Suzanne Kahn, a program manager at the Roosevelt Institute, a liberal think tank.

"I'm very glad that Mr. Buttigieg is worried that I have been too easy on upper-income people and the millionaires and billionaires," Sanders told MSNBC in an interview. "That I'm going to allow their kids to go to public colleges and universities, just, by the way, as they do go to public schools right now."

Warren has noted that Buttigieg's claim that the government shouldn't pay for college for affluent children rings hollow, since her plan is financed by a wealth tax of fortunes in excess of $50 million. She compared her tuition plan to the GI Bill after World War II that funded college for returning veterans, saying in an interview with the news outlet Iowa Starting Line that the program "not only helped millions of individuals, it also supercharged our economy."

Critics of Buttigieg's plan note that the current financial aid system is already income-based and that government support has failed to curb students' dependence on personal debt to finance the rising costs of college.

But key to Buttigieg's college plan is voters' sense of fairness that the wealthy should not be subsidized at public expense.

"You have people who are left of center wondering why the University of Virginia or Berkeley should be free to the children of the affluent," said Jason Delisle, a resident fellow at the conservative American Enterprise Institute whose research focuses on college financing.

College costs have climbed as a degree has essentially become a requirement for entering the job market — with all the net job gains over the past 12 months going to college grads, even though this group makes up just a third of the U.S. population, according to Labor Department figures. The result is that student debt has become the price of admission to the U.S. economy, imposing financial burdens that have been associated with delays in home-buying, marriage and having children.

"Higher education is an engine of enabling citizens to achieve in life independently of where they started — having those engines in life makes society healthy," said Marshall Steinbaum, an economist with the University of Utah.

In an economy that depends on consumer spending, the risk is that the student debt buildup after the Great Recession has suppressed spending and saving by the more than 40 million Americans who borrowed for school — and that hurts overall economic growth.

Concerns about student debt have mounted as college grads have become more likely to support Democrats. In 2018, Democrats nationwide won college graduates by 14 points, according to AP VoteCast, a nationwide survey of midterm voters. White college graduates had until recently been a reliable group of voters for Republicans, but they're now increasingly tilting toward Democrats.

But proposals for free college also carry risks as the issue splits along partisan and generational lines.

While nearly three-quarters of Democrats consider free public college tuition a good idea, just about 2 in 10 Republicans do, according to a July poll from NPR/PBS NewsHour/Marist. Less than half of Americans older than 45 support the idea, while 62% of Americans younger than 45 favor free college tuition.