Question

By criticizing the party that is in power, the party that is out of power serves a _____ function

Answer 1

The party out of power serves a "watchdog" function. They try to protect the people by criticizing the party in power which makes people ask questions about the good intentions of their government.

Answer 2

I believe the answer is: Watchdog

In our government, the Watchdog  would always take a opposition stand for all the regulations that proposed by the winning party.
By doing this, the government could evaluate the points that might overlooked by the winning party and evaluate potential risk for the plan.