The gap in bachelor-degree attainment between the nation's richest and poorest students by age 24 has doubled during the last four decades.

A study released Tuesday by two education organizations finds that the percent of students from the lowest-income families earning a bachelor's degree has inched up just 3 points since 1970, rising from 6 to 9 percent by 2013.

Meanwhile, college completion for students from the wealthiest families has risen dramatically, climbing from 44 to 77 percent.

The report comes amid renewed debate on college affordability spurred by President Barack Obama's proposal to make two years of college free. If adopted in every state, the proposal would benefit a projected 9 million students each year. It would cost taxpayers an estimated $60 billion over 10 years.