A “party line” is the ideology or the agenda of a political party. The party line consists of most core tenets of a party, as well as anything they are attempting to accomplish.
The phrase is most often used in terms of a party-line vote. A party-line vote is when most or all politicians vote with their party on a proposal. For example, if there are 52 Republicans and 48 Democrats in the Senate, a party line vote might be 51-49 Republican.
Forbes: “Party-line voting has become the new normal. As recently as the early 1970s, party unity voting was around 60% but today it is closer to 90% in both the House and Senate.”