Question

What does it mean to vote on party lines?

Answer 1

to vote in a manner that is consistent with the official policy or opinion of one's political party

Answer 2

A party-line vote in a deliberative assembly (such as a constituent assembly, parliament, or legislature) is a vote in which a substantial majority of members of a political party vote the same way (usually in opposition to the other political party(ies) whose members vote the opposite way).