Community colleges began in the early 20th century, known then as junior colleges, and expanded rapidly in the 1960s. They became not only steppingstones to four-year colleges but also places that trained students for specific jobs, like nurses, paralegals or engineering technicians.

The largest, each with tens of thousands of students, include Miami Dade College; Northern Virginia Community College; City College of San Francisco; Lone Star College, in Houston; and Kingsborough Community College, in Brooklyn.

The report describes a network of federal and state educational policies that has failed to keep pace with the increasing enrollment of lower-income students in higher education. The largest federal financial aid program — Pell grants, which go to lower-income students to offset tuition — does relatively little to help community colleges because their tuition tends to be low.

“In the 20th century, going to college was not necessary for getting a job in the middle class,” said Eduardo J. Padrón, the president of Miami Dade College and a co-chairman of the 22-member committee. “But in today’s job market, if you don’t have a postsecondary credential, you can’t get a job that lets you achieve the American dream. It keeps you in a cycle of poverty.”

Community colleges and four-year colleges have both suffered in recent years from state budget cuts, said Sandy Baum, a senior fellow at the George Washington University Graduate School of Education and Human Development. But four-year colleges have made up some of the shortfall through tuition increases, while community colleges have not increased tuition as rapidly.

The financing gap, Ms. Goldrick-Rab said, “is contributing to really appalling completion rates.”

The report recommended a series of policy changes, including more transparency about who benefits from federal education spending; more outcome-based financing, to reward colleges that do the best job with challenging students; and programs to make community colleges economically diverse. Community colleges could create more honors programs, including classes for high school students, and four-year colleges could set aside more slots for community college transfers, the authors said.

Other research has found that poor students tend to fare worse, all else equal, when enrolled in a school made up mostly of poor students. Yet over the last generation, higher education appears to have become more stratified.