Many Democratic candidates for president want public college tuition to be free.

Joe Biden proposed that when he was vice president. Sens. Bernie Sanders (I-Vt.) and Kamala D. Harris (D-Calif.) co-sponsored a bill to make it happen. Sen. Elizabeth Warren (D-Mass.) has free tuition in her higher education platform.

John Mullane, a teacher, community-college counselor and scholar, has shown me he knows more about this issue than they do. He has worked on this for many years and started a Connecticut company named College Transfer Solutions.

“States can make college as free as they want,” he said in a recent letter to the House Committee on Education and Labor, “but if they don’t have a system in place to help students get through these institutions and graduate on time with a college degree that allows them to go directly into a good job, or to fully transfer the credits to a bachelor’s degree, they are doing more harm than good.”

Several presidential candidates have focused on eliminating tuition for students in two-year community colleges. But in some ways, Mullane said, that has already happened. If your family income is low enough, you are eligible for a Pell Grant. It pays $6,095 a year. The average community-college student needs only $3,660 for tuition and fees, leaving $2,435 for expenses such as books and transportation.

Sadly, there aren’t nearly enough Pell Grants for everyone. But the cost of community college is not the most important issue. Mullane wishes the candidates would take their eyes off tuition money for a moment and look at wrongheaded practices that stymie student progress.

Free tuition will probably mean less money to hire professors and advisers. It will increase the number of students on campus who can’t find space in ­courses required for their major.

“The main problem is that students do not have a clear path through these institutions that will allow them to fully transfer their credits and apply them to a bachelor’s degree at a public four-year institution,” Mullane said.

“Around 80 percent of community-college students who transfer do not complete a degree before transferring to a four-year school,” he said. “Many of these students . . . pay between three times up to 10 times more in tuition to take classes they couldn’t get at their community college because these courses were not offered or wouldn’t transfer.”

I have an unscientific suspicion that humans are less likely to work hard for something they or their families aren’t paying for. I wonder if no tuition would mean less study and, thus, less success. But people who have had full scholarships, such as the woman I married, tell me I’m an idiot, so I will stick with Mullane’s well-researched conclusions.

He shows the problem derives, in part, from colleges hiding what they are doing. This echoes a report from KIPP, the charter school network, that says American higher education is keeping low-income students from having the information they need to make wise college choices.

Mullane wants colleges to release more data on how many transfer students get degrees. The federal government reports only the graduation rates of first-time, full-time students who graduate from the same institution within three years of enrolling at a community college or within six years of enrolling at a four-year college.

He argues that four-year schools should be required to disclose the percentage of community-college credits they accept. Those states with laws governing transfer credits should start enforcing them, he says. They should also eliminate noncredit remedial courses — a big waste of students’ money — and instead provide more support for students who struggle in courses taken for credit — something that the best-run colleges are already doing.

Forty-nine percent of students who complete bachelor’s degrees are community-college transfers. That includes newscaster Jim Lehrer, Tom Hanks, my mom and Jaime Escalante, the teacher who inspired me to be an education writer. We should be talking less about how much money to spend on them and more about how to spend it.