The Democratic presidential candidates’ competing higher education plans got valuable airtime in the December debate. The fact that all of them are making affordable higher education a campaign issue is good news, but the fine print makes all the difference.

Any proposal to spend federal dollars on higher education should concentrate on those who actually need this help, for otherwise it is money wasted. On that score, neither Elizabeth Warren nor Bernie Sanders makes the grade. They are engaged in a bidding war over who is the most generous -- their proposals cost more than the entire 2009 stimulus package -- but there’s too much pandering to the rich and too little smart policy.

Both candidates would eliminate tuition for everyone, but those atop the income ladder do not need that kind of help. It is perverse to make college free for the scions of the super-rich, since they are the very individuals whose taxes would go up, under both candidates’ plans, to pay for their campaign promises.

These candidates’ debt relief plans are off base for the same reason -- they are a bailout to those who least need the money. A recent report from the American Enterprise Institute, a conservative think tank, shows that the number of college borrowers from rich families nearly doubled in two decades. In 2015-16, they were just as likely to borrow money as students from poverty-level households, and their loans were almost twice as much as their low-income classmates. Yet it’s hard to worry about the debt burden they have chosen to carry.

What’s more, about 40 percent of college debt is from graduate studies -- social work students at the University of California, unlikely candidates for six-figure incomes, borrowed, on average, $109,486 for a master’s degree. While the university ought to be ashamed of itself for the fact that sky-high tuition meant the typical social work master's students borrowed more than $100,000, I’m not worried about the bills corporate lawyers and hedge fund managers piled up in grad school.

Joe Biden’s big idea -- make community college, but not a four-year school, free for all -- sounds more plausible. Barack Obama started to implement it, and Hillary Clinton embraced it in her 2016 campaign. Yet this well-intentioned strategy may actually make things worse.

How so? The rationale of the Biden plan is that the offer of free tuition will entice high school graduates to choose community college, and then, associate degree in hand, transfer to a four-year school. In Tennessee, which pioneered that approach, community college enrollment did indeed increase. But only 22 percent of those students received an associate degree in two years, and nearly half dropped out. Had they attended a regional university like Middle Tennessee State instead, they would have a 50-50 chance of earning a bachelor’s degree. You can do the math.

Pete Buttigieg has weighed in with a more nuanced approach. His plan lowers tuition and fees on a sliding scale, with free college for those whose families earn up to $100,000 and subsidies for families earning up to $150,000. A $1,000 increase in Pell Grants, awarded to poor families, would help cover their living costs. This often-ignored cost of going to college merits attention, because these expenses may well be greater than the cost of a bachelor’s degree.

For Sanders, Warren and Biden, money -- more precisely, the lack of money -- is the problem. While that is the case for some students, it is far from the entire story.

Only half the students who enroll in a public university graduate in six years, and less than a third earn a community college degree in four years. That hair-on-fire fact conceals major differences among institutions. Data compiled by the Education Trust shows that the graduation rate at universities with the same admissions criteria can vary by as much as 30 percent. The opportunity gaps are even bigger. At institutions like Rutgers University at Newark and the University of Central Florida, black and Latino students are nearly as likely to graduate as their classmates, while at the University of Toledo, 14 percent of black students and 36 percent of Latino students graduate compared to about 53 percent of white students.

Similar graduation gaps show up among community colleges. Although these institutions accept anyone with a high school diploma, the best of them graduate more than half their students, while the worst award degrees to fewer than 10 percent.

No one is held accountable for this sorry state of affairs. No one is fired because the dropout rate is unacceptably high.

What makes the situation scandalous is that we have the tools to move the needle on student success and close the opportunity gap. What is missing is the will to act. When a college makes student success its top priority, it can readily spot -- and remove -- the roadblocks to graduation. What’s more, as institutions such as Georgia State University and Long Beach State University have proven, boosting the graduation rate doesn’t have to cost the moon. In my book, The College Dropout Scandal, I detail the strategies of these and other universities that have prioritized student success.

Alone among the candidates, Buttigieg’s plan addresses this dropout problem. In exchange for federal and state free tuition investments, public universities would have to spend as much as a quarter of this money on testing promising ideas or adopting proven practices. Some of those practices might include, for instance, adding counselors and advisers, who offer students “we have your back” academic and personal support, or providing mini grants that enable students to weather financial crises.

“Free college” makes for a catchy bumper sticker. Buttigieg’s plan does not lend itself to sloganeering -- try “free tuition for some, reduced tuition for others, strategies to boost student success” on for size -- but it is the best by a long shot. Delivering income-based help that makes college affordable and prodding the institutions to do a better job of getting their students across the finish line would dramatically alter the landscape of American higher education for the better.