Other forms: party lines
A party line is a group's official opinion, policy, or agenda. Political parties tend to be happiest with legislators who closely stick to the party line, voting the way the party wants them to.
It's usually political parties and organizations that are described as having party lines — although other groups, like businesses, clubs, or religions, often have party lines they expect members to follow. You'll frequently hear the phrase "toe the party line," which means "stick to the official party message," something that commonly happens when a politician gives a speech. Long ago, a party line was also a telephone landline that was shared by several families.