WASHINGTON — As part of its effort to expand private health care, the Department of Veterans Affairs is exploring the possibility of merging its health system with the Pentagon’s, a cost-saving measure that veterans groups say could threaten the viability of VA hospitals and clinics. VA spokesman Curt Cashour called the plan a potential “game-changer” that would “provide better care for veterans at a lower cost to taxpayers,” but he provided no specific details. Griffin Anderson, a spokesman for the Democrats on the House Veterans Affairs Committee, said the proposal — developed without input from Congress — would amount to a merger of the VA’s Choice and the military’s TRICARE private health care programs.