A person stands in the window of the library on the Georgia State University campus Monday, Feb. 22, 2016, in Atlanta. Georgia pioneered a program to make college accessible to anyone in the state with a B average, but such programs have potential pitfalls. (AP Photo/David Goldman)

College affordability looms large on the campaign trail this presidential election, as candidates in both parties try to appeal to young voters and their parents, who are increasingly frustrated with runaway tuition prices. Bernie Sanders has attracted the most attention on this topic because he has been consistent on one promise throughout his campaign: to make college free.

The simplicity and universality of his message is reminiscent of a promise of free college from another politician more than two decades ago: Zell Miller. Miller was running for governor of Georgia in 1990, and he was worried that the best and brightest students were leaving the state for college and never coming back.

A Marine Corp veteran, Miller — along with his political consultants, including James Carville — hatched a plan, what they called a G.I. Bill for Georgia. The result was the HOPE Scholarship (Helping Outstanding Pupils Educationally), which would pay for tuition and fees at a public college in Georgia and partial tuition at a private college to any student who graduated high school with a B average. The scholarship would be funded with another centerpiece of Miller’s campaign for governor, a new lottery.

In an op-ed for The Washington Post, Democratic presidential candidate Bernie Sanders explains legislation that would drastically change higher education in America. (The Washington Post)

Miller was elected governor, and in 1993 Georgia pioneered a revolution in how states delivered financial aid. Rather than base their limited aid dollars on financially needy students, money would be available to anyone with a B average. The idea swiftly gained traction elsewhere, and within a decade, 13 states had adopted similar broad-based scholarships modeled after Georgia’s HOPE program.

Steve Lopez, a columnist for the Philadelphia Inquirer, wrote in 1996 that HOPE “is the kind of thing you look at half in amazement and half in anger, and wonder why your own bonehead state didn’t think of it.”

But how the broad-based college scholarship programs have evolved in the states since the 1990s provides three important lessons to politicians who want to extend the idea of free college at the federal level:

1. Free college mostly helps students who would get a degree anyway.

The former chancellor of Florida’s university system, Charles Reed, frequently told a story about an orthopedic surgeon who approached him once to compliment him on the quality of the state’s public colleges. The surgeon’s twin daughters went to the University of Florida on the state’s Bright Futures scholarship.

“That’s when I knew these scholarships were wrong,” Reed told me. “Here was a man who could easily afford tuition at the University of Florida and his daughters were getting a free ride from the state.”

The U.S. has a big problem getting poor kids to graduate from college, not students from middle-income and affluent families. More than 80 percent of students from families in the top income bracket in the U.S. now hold a bachelor’s degree by the age of 24, compared to just 8 percent for those in the bottom income bracket.

Yet the merit scholarship programs in the states disproportionately help students from middle and upper income families. In Florida, for instance, only one-fifth of the tuition waivers go to students whose families earn less than $30,000 annually.

Low-income students also are more likely to lose the scholarship because they encounter minor obstacles in college and they don’t want to ask for help, or don’t know how to. Nearly a third of low-income students in Florida fail to keep their scholarship for their sophomore year and many end up dropping out.

Free college might help those poor students whose biggest hurdle to graduation is financial. But researchers have identified plenty of other reasons beyond money as to why low-income students fail to earn a college degree, including academic difficulties and a mindset about fear of failure.

2. A post-high school education is critical in today’s economy, but not all high-school seniors are immediately ready for college.

The HOPE program and its copycats have redefined the very idea of merit. Public flagship universities in the states with merit scholarship programs are overrun with award recipients. For example, in the early days of Florida’s program, 99 percent of in-state freshmen at the University of Florida received the state’s Bright Futures award. But in the same year, 10 percent of award recipients were placed in remedial classes because they arrived on campuses not prepared to do college-level work.

The story of “merit scholars” who require essentially do-over high school courses is similar in other states that have the scholarships. Several studies have found that an unintended consequence of the scholarships is grade inflation, as teachers and professors are under pressure to give good grades so that students get the awards and keep them while in college.

The Sanders plan would make college tuition free at any public institution so long as the student can get in. As the merit scholarship programs have shown in the states, just because students can get accepted doesn’t mean they are ready for college.

3. Free tuition doesn’t reduce the cost of college.

Perhaps the biggest problem with state scholarship programs is that they didn’t put a lid on tuition. The cost of college continued to rise; someone else just footed the bill, mostly those who played the lottery.

In some states with the scholarships, the legislature sets tuition. To rein in the increasing cost of the awards, lawmakers in some states agreed to cap tuition. That led colleges in those states to look elsewhere for revenue from students, so they increased fees and other charges that the scholarships didn’t cover. There is nothing in the Sanders plan to stop institutions from following the same pricing strategy.

In other states where colleges controlled tuition rates, the cost of the scholarships became unsustainable over time as tuition prices continued to rise and lottery revenues didn’t keep pace. So states started to trim the program’s benefits — paying for partial tuition, limiting the number of semesters students could get the awards, or shifting the grade requirements needed to keep the scholarship.

As Andrew Kelly, of the American Enterprise Institute, has written about free college at the federal level, capping tuition at zero “limits college spending to whatever the public is willing to invest. But it does not change the cost of college.”

The idea of free college might sound great to students and their parents worried about the staggering cost of a degree, but the experience of a dozen states during the past decade shows that free tuition fails to change the college-going patterns of low-income students and quickly becomes an entitlement for those students who need it the least.