Is it a tax or is it not?  The Supreme Court has said yes and thus is constitutional.  In 2009, President Obama said that it wasn't repeatedly.  Republicans said that it did not fall under the Commerce Clause for the Congress to pass such legislation, and therefore, wasn't constitutional.

The United States’ Military and Department of Defense make up a huge portion of our country’s expenditures and has greatly influenced our government and citizens. Many Americans, however, don't know the facts. Our nation considers the security of our citizens and our nation to be of the utmost importance in terms of our government's priorities.