The United States is on the cusp of a major debate in both political parties over the country’s role in the world, but so far the presidential candidates have not made clear their own priorities in a time when defense spending is unlikely to grow, a panel of national security experts agreed Tuesday. Jim Thomas, a distinguished senior fellow at the Center for Strategic and Budgetary Assessment, said today at the Center for Strategic and International Studies that “there’s always going to be hard limits as to how far [an incoming president] can go” in changing national security strategy – debt payments, rising entitlement costs, treaty constraints, a balky Congress and evolving international challenges.