There are two critical things to know about community colleges.

The first is that they could be the nation’s most powerful tools to improve the opportunities of less privileged Americans, giving them a shot at harnessing a fast-changing job market and building a more equitable, inclusive society for all of us. The second is that, at this job, they have largely failed.

When President Obama stood at Pellissippi Community College in Knoxville, Tenn., last month and offered every committed student two years’ worth of community college at the government’s expense, he focused on the first point.

With open enrollment and an average price tag of $3,800 a year for full-time students, community colleges are pretty much the only shot at a higher education for those who don’t have the cash or the high school record to go to a four-year university. And that’s a lot of people: 45 percent of the undergraduate students in the country.

They are “essential pathways to the middle class,” Mr. Obama said. They work for parents and full-time workers, for veterans re-entering civilian life, and for those who “don’t have the capacity to just suddenly go study for four years and not work.”