As state legislatures prepare to meet in January, lawmakers across the country are girding for a battle over whether to sign on to the health-care law's expansion of Medicaid."This is the number one issue," said state Sen. Michael Lamoureux (R), incoming president of the Arkansas Senate. "And in 10 years this is by far the most difficult one we've ever dealt with." The national implications loom just as large. No provision is more central to achieving the health-care law's aim of extending coverage to the uninsured than its expansion of Medicaid.