Public colleges and universities used to depend mostly on state funding to keep their doors open, but they are increasingly relying on money from families paying ever-rising tuition.

Even though states increased higher education spending in 2014, tuition accounted for nearly half of public school revenue for the third year in a row, according to a new report from the State Higher Education Executive Officers Association.

Many advocates say that college has become unaffordable for many families because of lower funding from state governments.

There has been a seismic shift in the way public colleges are funded in just the last 26 years. In 1989, tuition made up a quarter of the total education revenue at state universities. By last year, those dollars accounted for 47.1 percent of the money schools need to educate students, the report said. While that's down from the all-time high of 47.7 percent in 2013, it still represents significant change in funding.

The reliance on tuition dollars widely varies from one state to another. Only about 15 percent of education revenue is sourced from tuition in Wyoming, a state with a lot of oil money and a small population. By comparison, tuition accounts for 85 percent of education revenue in Vermont, a state with a high proportion of private schools and out-of-state students.

Twenty-eight states get more than half of their higher education revenue from tuition, with 15 of them collecting more than 60 percent of the money they need from student dollars, the report said.

Public universities have grown more reliant on tuition dollars in the wake of the 2008 economic recession. The downturn hurt state budgets and legislatures responded by cutting higher education funding.

At the same time, there was an influx of people enrolling in school, placing added pressure on already stretched school budgets. The number of students enrolled in public colleges rose by 20 percent from the 2002-2003 school year to 2011-2012, according to a recent Government Accountability Office report.

In the face of soaring enrollment and declining state dollars, universities raised tuition to make up for the funding shortfall. And while the federal government stepped in with increased grant aid for students, the money has failed to keep pace with the cost of college.

Coming out of the recession, states began slowly spending more money on higher education, though not as much as they did before the financial markets crashed. State and local governments spent an average $6,552 per student in 2014, a 5.4 percent increase from the prior year, but 13 percent less than five years ago, according to the report.

Andy Carlson, a senior policy analyst at the association, explained that the improvement in state support is indicative of the economic recovery and a decline in enrollment. State appropriations can stretch a little farther when there are fewer students attending.

Enrollment across the country peaked at 11.6 million students in 2011 and has slipped a little every year since then, the report said. It's common for college enrollment to fall as the economy and job market improves. But some public universities imposed enrollment caps to offset continued budget cuts coming out of the recession. And would-be college students who couldn't afford skyrocketing tuition likely passed on the opportunity to go, the report said.

Still, the number of people enrolled in public colleges and universities full time last year was almost 50 percent higher than in 1989, and about 15 percent higher than 2004.