This interesting question and the following was written by John Chandler, director at chandlersearch.net

A reporter recently asked House Speaker Nancy Pelosi (D–CA) where in the Constitution Congress is given the authority to force Americans to buy health insurance, she responded, “Are you serious? Are you serious?” Responding to a follow-up question to this non-answer, Pelosi’s press secretary said, “That is not a serious question.” (she didn’t answer because she and ALL of Congress knows that it is NOT Constitutional)

We think otherwise at The Heritage Foundation. So should all Americans who value the liberties which our Constitution protects.

And once the mandate question is thoroughly examined through the lens of the Constitution’s original meaning, the answer is inescapable: it is not constitutional.

“For those with a traditional understanding of the Constitution as a charter of liberty (as opposed to the ‘living version’), the list of Congress’ powers in Article I, Section 8, grants it no authority to require any such thing, “writes Heritage expert Bob Moffit. To defend their unprecedented expansion of federal power, Obamacare’s proponents rely upon excessively broad interpretations of Congress’ powers — namely the powers to regulate interstate commerce and impose taxes.

Commerce and taxing power don’t support mandate

In a recent legal memorandum, Heritage legal scholar Todd Gaziano, joined by Georgetown law professor Randy Barnett and Nathaniel Stewart, LLP, argues that the individual mandate is both unconstitutional and unprecedented.

Neither the power to regulate commerce nor the power to impose taxes grants Congress the power, they write, to “mandate that an individual enter into a contract with a private party or purchase a good or service and…no decision or present doctrine of the Supreme Court justifies such a claim of power.”
Though many on the Left overlooked it, the nonpartisan Congressional Research Service even recognized the constitutional obstacles to the individual mandate:

Whether such [an individual mandate] requirement would be constitutional under the Commerce Clause is perhaps the most challenging question posed by such a proposal, as it is a novel issue whether Congress may use this clause to require an individual to purchase a good or a service.

The Supreme Court has long held that there are limits to Congress’ power under the Commerce Clause. An individual mandate would require stretching these limits. Although this has been done in the past, “the current Supreme Court is unlikely to stretch the commerce power further than it already has,” explain Gaziano, Barnett and Stewart.
Congress’ power to “lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defense and general Welfare of the United States….” also does not validate an individual mandate. The Constitution requires that a tax be apportioned on the basis of the census population, and not vary based upon factors such as the financial condition of the state’s residents. “[But] this [constitutional] requirement will be impossible to meet based upon the variety of exceptions provided in the mandate,” write Gaziano, Barnett and Stewart.
Interpreting the Constitution to support an individual mandate would open the door for future abuses, as Heritage’s Conn Carroll explains:
If the individual mandate is Constitutional, then Congress could do anything. They could: require us to buy a new Chevy Impala each year to support the government-supported auto industry; require us to buy war bonds to pay for the Iraq and Afghan wars; require us to grow wheat (10 bushels each), or pay someone else to grow your share; require us to buy whatever they want.