Breaking: Obama admin unilaterally declares law passed by Congress 'unconstitutional'

The law in question is the Defense of Marriage Act, Section 3. The president and attorney general, in a letter today, declared it unconstitutional and state that the government will no longer enforce it. Congress passed DOMA in 1996, 85-14 in the Senate and 342-67 in the House, and President Clinton signed it into law.

Does a president have the power to unilaterally declare laws passed by Congress and signed by his predecessors “unconstitutional?” This strikes me as setting an extremely dangerous precedent.