I could give a very long analysis of why the mandate to purchase health insurance under the Affordable Care Act is constitutional. But, I don’ t think there is mucn need to do so. Our judiciary has become so politicized that there is no telling what the Supreme Court would do and good reason to fear that the Republican majoritiy will do the wrong thing just because of the partisan makeup of the court. But from the point of view of constitutional law, no long analysis is reallyi needed. There are three good reasons that the mandate is constitutional aand each them is fairliy obvious.
1. The commerce clause has been broadly interpreted to allow Congress to regulate almost any industry and to subsidize the purcharse of any good. The mandate is a critical part of a new regulatory and subsidiy scheme. We can’t sensibly impose insurance company regulations that force companies to insure anyone and not to charge more for those without preexisting conditions because people will have an incentive to only get insurance when they get sick if they can get insurance whenever they do get sick. If lots of people do that, the cost of insurance for those who have would rise dramatically undermining the whole regulatory scheme. So the mandate is a necessary means to make the kinds of regulation and subsidy that Congress clearliy has a right to carry out.
2. Congress clearly has the power to tax people in order to provide them a benefit. Does anyone have a doubt about the constitutionality of Social Security or Medicare? The ACA can be understood as is just another means of attaining the same end. As I’ve argued against single payer fanatics, there is no deep conceptual difference between requiring everyone to purchase subsidized insurance and taxing people to give them health care.
3.Congress has the power to fine or tax people when they impose a burden on others. People without insurance impose a burden on others because no one is denied emergency care in the US and those of us who have insurance pay for the medical care of those 
who do not.. The mandate can be understood—and in fact is defined in the legislation–as a tax that recovers some of those costs.
I can’t speak to federal precedents right now, but one example that comes to mind is the mandate to have auto insurance. It is true that people who don’t own or operate vehicles are not required to have this insurance, but I would defy anybody who claimed that there exist people who never in the lives required medical care. So the “pool” is effectively universal.