His basis is that this practice, it’s not a rule, has happened in the last year of a term WHEN THE SENATE IS CONTROLLED by the other party than the president. His distinction is that there is no reason to expect that a Senate of the same party would exercise its discretion not to take up the nomination. Makes sense to me, and the rats could teach us volumes about obstructing judicial nominations of a president of the other party, just as they could teach us about ramming home your own president’s nominees, a la Reid’s nuclear option
In other words his basis for this is whatever he wants his basis to be. It's open for change to suit whatever the circumstances are.