Sen. Richard Durbin is asking the Department of Defense to investigate how the University of Phoenix markets itself to military personnel and veterans, and is urging that the for-profit college be barred from military bases until a probe is complete.

The Illinois Democrat, long a critic of recruiting tactics employed by for-profit colleges, expressed outrage in a letter to Defense Secretary Ash Carter dated June 30, calling the military activities a violation of “the spirit of” a 2012 executive order intended to protect service members from aggressive college recruiters.

A spokeswoman for Mr. Durbin said Thursday he hadn’t yet received a response to his letter.

The recruiting practices of the University of Phoenix, owned by Apollo Group Inc., were recently the subject of a critical story by the Center for Investigative Reporting, a nonprofit news site. Like many other companies and organizations, the university sponsors morale, welfare and recreation events at military installations nationwide.

“The only reason we would be on post is because we were invited on” by post commanders, said retired Army Maj. Gen. James “Spider” Marks, who now heads University of Phoenix’s College of Security and Criminal Justice.