President Trump has tasked three Republican senators with coming up with a replacement for Obamacare if courts strike it down. It’s a prudent contingency plan. Republicans should, for that matter, advance their own health-care plan even if the lawsuit fails. They cannot prevent Democrats from attacking them over health care by abandoning the issue.Obamacare’s key innovation was not the subsidization of Americans’ health-insurance purchases, an enterprise in which the federal government had been engaged for decades, albeit on a somewhat smaller scale. It was the centralization of health-insurance regulation in Washington, D.C. It is that centralization, and accompanying curtailment of choice and raising of costs, that Republicans tasked with replacing Obamacare should now work to undo.