When Congress required most Americans to obtain health insurance or pay a penalty, Democrats denied that they were creating a new tax. But in court, the Obama administration and its allies now defend the requirement as an exercise of the government’s “power to lay and collect taxes.” And that power, they say, is even more sweeping than the federal power to regulate interstate commerce. Administration officials say the tax argument is a linchpin of their legal case in defense of the health care overhaul and its individual mandate, now being challenged in court by more than 20 states and several private organizations. Under the legislation signed by President Obama in March, most Americans will have to maintain “minimum essential coverage” starting in 2014. Many people will be eligible for federal subsidies to help them pay premiums.