They asked McCullough to complete the review of the surveillance programs by the end of next year and release a public summary of his findings.
"We have urged appropriate oversight of these activities long before the problems with the implementation of these [Foreign Intelligence Surveillance Act] authorities became public," the senators wrote. "We believe it is important for your office to begin this review without further delay."
They noted that many agencies have their own inspector general, but argued that only McCullough is in a position to provide a comprehensive report on the government's surveillance activities.
The Washington Post published an internal NSA audit last month showing that analysts had illegally obtained private communications thousands of times in recent years. The government also released a 2009 classified court opinion showing that analysts had improperly accessed data on thousands of phone numbers over several years.
The NSA has insisted that most of those privacy violations were accidental and that it has implemented tougher internal oversight.
Many lawmakers have also expressed shock to learn that the NSA has been using a provision of the Patriot Act to collection data on all U.S. phone calls. The provision, Section 215, authorizes the NSA to collect records only if they are "relevant" to a terrorism investigation.