This is the time when a politician is nearly done with his/her term and the time before they are reelected.
In this case, the lame duck vice-president would become president.
The incumbent is still the president, and the new one is the president elect.
It isn't "sitting duck" but lame duck. A lame duck is a person in office whose term of office is nearly finished.
lame duck
George HW Bush
lame duck
It is called "lame duck"
He is a lame duck.
It is called a "Lame Duck."
The period between election day (November 4th) and the inauguration of the next president (January 21st). The sitting-president is considered a "lame duck" since the president-elect will be strolling into the oval office within a couple of months.
A president about to leave office is referred to as a "Lame Duck" president.
Answerlame duck