Over the past eight years, high-stakes negotiations in Congress over the federal debt limit have repeatedly brought Washington to the verge of default. We were on opposite sides of these debates, as senior policy advisers to President Obama and Senate Republican Leader Mitch McConnell, and we continue to disagree about taxes and the proper size of government. Yet we both believe that the statutory debt limit has outlived its usefulness as a mechanism for restraining the size of the national debt. Or, put more precisely, we think that whatever residual value the debt limit may have is far outweighed by the risk that a…