The party line is the way of thinking, or
ideology you have to have to be a part of the
group. Generally, if you deviate from the party line there are harsh consequences if you do not mend your ways. Or, alternatively, group members might just be
condesending to the
rebel among them.
For example, the Republican party line could be generally defined as fewer taxes, less government, protected second amendment rights and a strong national defense. While the Democratic party line is generally social justice via affirmative action, gun control, higher taxes for the rich, environmental protection and less defense spending.
Please note that these are gross stereotypes just used to help a definition. I am sure people disagree with my characterizations. Do not take offense.