Details here, in a discussion on whether the executive branch is required to automatically defend all laws passed by congress, or only the ones the President agrees with:
Instead of requiring DOJ to defend the constitutionality of all federal statutes if it has a reasonable basis to do so, the new approach invests within DOJ a power to conduct an independent constitutional review of the issues, to decide the main issues in the case — in this case, the degree of scrutiny for gay rights issues — and then, upon deciding the main issue, to decide if there is a reasonable basis for arguing the other side. If you take that view, the Executive Branch essentially has the power to decide what legislation it will defend based on whatever views of the Constitution are popular or associated with that Administration. It changes the role of the Executive branch in defending litigation from the traditional dutiful servant of Congress to major institutional player with a great deal of discretion.
Here are the potential consequences:
If Congress passes legislation on a largely party-line vote, the losing side just has to fashion some constitutional theories for why the legislation is unconstitutional and then wait for its side to win the Presidency. As soon as its side wins the Presidency, activists on its side can file constitutional challenges based on the theories; the Executive branch can adopt the theories and conclude that, based on the theories, the legislation is unconstitutional; and then the challenges to the legislation will go undefended.