The party in power.
The US Republican party is represented by an elephant.
Andrew Fisher, Prime Minister of Australia three times, was a member of the Labour political party.
the party of Prime Minister Stephen Harper is Conservative.
Donald Dewar and he represented the Labour party.
It is the political party that loses/wins the election. The prime minister is the appointed leader of the party that wins. When a political party loses a general election then the prime minister steps down and the leader of the winning party becomes prime minister.
They get political followers and become head of a political party that runs him/her as prime minister.
Republicans
President William McKinley represented the Republican Party.
Janata Party
Julia Gillard is from the Australian Labor Party
Winston Churchill was the Conservative prime minister.
GOP is an acronym for Grand Old Party.