Government Mandated Health Insurance, is it Constitutional?

We can all agree that Healthcare Reform is necessary, and rising costs cannot continue. However, what has been proposed so far is not necessary to cover a minority of the population currently without health insurance. Why not simply let Insurance Companies offer insurance across state lines, this would be competitive, and bring down premiums. So far no group or organization has come forward to challenge the Governments right to force U. S. Citizens to buy something, because that is exactly what they are doing. What will be the next pretext to intrude on the lives of Americans?