Some of the nation’s biggest banks have closed their doors to students at community colleges, for-profit universities and other less competitive institutions, even as they continue to extend federally backed loans to students at the nation’s top universities.

Citibank has been among the most aggressive in paring the list of colleges it serves. JPMorgan Chase, PNC and SunTrust say they have not dropped whole categories, but are cutting colleges as well. Some less-selective four-year colleges, like Eastern Oregon University and William Jessup University in Rocklin, Calif., say they have been summarily dropped by some lenders.

The practice suggests that if the credit crisis and the ensuing turmoil in the student loan business persist, some of the nation’s neediest students will be hurt the most. The difficulty borrowing may deter them from attending school or prompt them to take a semester off. When they get student loans, they will wind up with less attractive terms and may run a greater risk of default if they have to switch lenders in the middle of their college years.

Tuition and loan amounts can be quite small at community colleges. But these institutions, which are a stepping stone to other educational programs or to better jobs, often draw students from the lower rungs of the economic ladder. More than 6.2 million of the nation’s 14.8 million undergraduates  over 40 percent  attend community colleges. According to the most recent data from the College Board, about a third of their graduates took out loans, a majority of them federally guaranteed.