If, as expected, the GOP picks up seats in both the house and the senate on Tuesday, we will not doubt hear a lot of garbage about "voter mandates," "referendum on the Obama presidency," and so forth.
It is worth noting that the midterm elections during a President's 2nd term of office almost always swing dramatically for the opposition party. This is not unusual:
1950 Elections (2nd term for Truman):
House GOP +28
Senate GOP +5
1958 (2nd term for Eisenhower)
House Dem +49
Senate Dem +16
1966 (technically 2nd term for LBJ)
House GOP +47
Senate GOP +3
1974 Elections (Nixon, although elections took place after his resignation)
House Dem +49
Senate Dem +3
1986 (Reagan)
House Dem +5
Senate Dem +8 (and took majority)
1998 (Clinton)
House GOP -5
Senate Unchanged
The 1998 elections are the only exception to this trend since WWII
2006 (Bush)
House Dem + 31
Senate Dem +6