A state-level campaign to rein in the federal government by calling an unprecedented convention to amend the U.S. Constitution is gaining steam, picking up support from two high-profile Republicans as more states explore the idea. Coburn, a legendary government-waste watchdog, announced this week that he has joined the effort by becoming a senior adviser for the group Convention of States Action, which wants states, not just Congress, to pass constitutional amendments. Article V of the Constitution states amendments can be ratified either by Congress or by states if two-thirds of them petition Congress to call a convention. Then, any amendment...