RICHMOND — Attorney General Ken Cuccinelli II will not defend one of Gov. Robert F. McDonnell’s marquee education reforms against a looming legal challenge, saying he believes that legislation allowing the state to take over failing schools is unconstitutional.

Cuccinelli (R) said he supports the goal of the Opportunity Educational Institution, created by the General Assembly this year to take control of persistently failing schools. But he also said his office cannot defend it against a lawsuit that the Virginia School Boards Association and the Norfolk City School Board are preparing to file in Norfolk Circuit Court.

Cuccinelli’s letter was dated Aug. 27, the same day that McDonnell (R) traveled to a Norfolk middle school to tout the reform measure and vow to fight the threatened suit.

“Attorney General Cuccinelli certainly supports efforts to turn failing schools around,” spokesman Brian J. Gottstein said. “However, his legal analysis shows that, unfortunately, this law goes about it in a way that is unconstitutional. The Virginia Constitution states – and the courts have affirmed – that the supervision of public schools must remain with their local school districts.”

Cuccinelli, who is running to succeed McDonnell, has broken with his fellow Republican before, most notably on a $1.4 billion-a-year transportation-funding overhaul that McDonnell hailed as a bipartisan compromise and Cuccinelli called a “massive tax increase.”

This split comes as Cuccinelli’s campaign has sought to fight back against claims from Democratic rival Terry McAuliffe that he has let politics, rather than the law, guide his decisions as attorney general. Democrats often say that he “bullied” the state board of health into imposing strict building standards on abortion clinics, warning members last year that his office would not represent them and that they could be personally liable for legal bills if they did not reverse a decision to grandfather existing clinics.

Cuccinelli’s campaign has often noted that he opposed the transportation plan as a matter of policy, but also found a way to correct constitutional problems that made the law vulnerable to a court challenge. Cuccinelli’s office cast his stance on the school take-over legislation in similar terms.

“This is a purely legal issue,” Gottstein said. “If the attorney general’s analysis shows that a law is unconstitutional, he has a legal obligation to not defend it.”

McDonnell’s office said it was not deterred by Cuccinelli’s letter, which was first reported by the Richmond Times-Dispatch. The governor intends fight the anticipated lawsuit with help from a special counsel, which Cuccinelli’s said McDonnell was entitled to hire in place of the attorney general’s office.

“According to the Supreme Court of Virginia, it is well established that all actions of the General Assembly are presumed to be constitutional,” McDonnell spokesman Tucker Martin said. “Nevertheless, Virginia law provides for circumstances in which the attorney general determines he is unable or incapable of rendering legal services, including defending legislation passed by the General Assembly, and in such a situation the law provides for the appointment of special counsel. We look forward to working with special counsel to vigorously defend this important new law that will help ensure every child, in every community, gets a great education.”