Congress left Washington without keeping the interest rate on student loans from doubling today, soaring from 3.2 percent to 6.4 percent.

While that’s bad news for students and their families, it could be worse. Most people don’t borrow in July, and Congress will be back later this summer with time to find a compromise before school starts in the fall.

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What should be troubling, though, is the failure of the government to deal with interest rates, which should be the easy part of the college affordability problem.

A bigger cause of concern is the constantly rising principal of student loans, not the interest. College costs continue to rise, and students are expected to pay a bigger share of the cost of their education. That means many potential students either are priced out of starting college or forced to leave before acquiring a degree.

The interest rate is part of that, but just a small part. Since 1978, college tuition has increased by 1,120 percent, more than four times the general inflation rate. College costs have risen twice as fast as medical care and three times faster than housing.

Students entering the workforce loaded with debt could be the next credit bubble to threaten our economy. It’s already huge, totaling more than $1 trillion, more than all of the nation’s combined credit card debt.

In 2012, President Barack Obama proposed a way to control college costs. Grants (instead of loans) would be made to students, but they could be used only at colleges and universities that did not raise tuition and fees. Unfortunately that idea, very unpopular among college administrators, has not moved forward.

It’s not all the colleges’ fault. All around the country, states are contributing less to public higher education, putting more pressure on students and their families.

In a robust economy where graduates could expect to find high-wage jobs right out of school, this might not be so bad. But in our fifth year of slow economic growth following the biggest financial collapse in 80 years, there is no guarantee that college will start paying off immediately.

Still, a college degree remains the surest ticket into the middle class, and Congress should find ways to make it more affordable.

When Maine’s senators and representatives are in the state for this recess, we hope they will take the time to talk with students and families about the huge sacrifices they are making to invest in the economy of the future.

Congress should get serious about keeping student loans low, but they also should attack the real reason college is increasingly unaffordable.

It’s the tuition rates, not the interest rates, that are putting college out of reach.