This year, the Promise’s marketing has emphasized vocational college. Administrators hope marginal students will be less likely to drop out of such programs because they are shorter.

Income share agreements could lower costs and improve outcomes by tying loan amounts to objective judgments of how much the student is likely to earn from her degree. Educational quality could also benefit: Investors would presumably advance students money only for schools that were doing a decent job of teaching them. The risks are that some borrowers could end up paying far more under such a scheme than the current plan and that investors might not lend to students they consider too risky.

His administration cut out the middlemen by killing off the Guaranteed Student Loan Program, the one created under Presidents Johnson and Nixon that relied on banks, in favor of a direct loan program, in which money came from the Treasury. But the government’s loose lending policy, with few questions asked, remained in place. The Obama administration also heavily promoted income-based repayment programs, which set borrowers’ monthly payments at 10% of their discretionary income and then forgave a portion of their debt after 20 to 25 years of payments. This severed the link between the value of students’ education and how much they could borrow, providing a huge incentive for schools to raise tuition, since taxpayers would pick up more of the tab. Enrollment in these programs is one big reason that the government’s costs for student loans are exploding.

The voucher system, combined with a lack of government oversight, created perverse incentives: Colleges could raise money quickly by admitting academically suspect students while suffering little or no consequences if their students dropped out and defaulted on loans.

In particular, the system gave colleges an incentive to maximize the tuition they extracted from students and the federal taxpayer by boosting fees and enrollment, which meant relaxing admissions standards.