In recent years, traditional positions on federalism have changed somewhat, with liberals arguing for and conservatives arguing for on issues like medicinal marijuana and gay marriage.

1.  States Rights

2. Strong  National Power

Explanation:

It is always believed that, one of the fundmentals of Liberals in the United States is to let the Strong National power exists, so that any decision made, will be binding across the nation without some states being exempted.

While, conservatives are often characterized by holding old belief and system, while letting each of the states to determine for themselves.

However, in this case, it appears the traditional positions on federalism have changed somewhat,  with liberals arguing for STATE RIGHTS and conservatives arguing for STRONG NATIONAL POWER on the issues like medicinal marijuana and gay marriage.  

answer; ///i believe that the correct answer is because the house and senate frequently pass different versions of the same bill;