Answer:
Act as a "watchdog" over the party in power.
Step-by-step explanation:
A key role of a political party that is out of power is to "Act as a "watchdog" over the party in power."
To act as a watchdog is to keep the party in power on their toes constantly. Thereby making the party in power more accountable and quickly see their lapses.
Also, when a party out of power acts as a watchdog, it helps them seek more support against the party in power.