How has party-line voting in congress changed in the past few years?

It has increased noticeably. A party-line vote is one on which a larger part of Republicans vote one way and a lion's share of Democrats vote another. A bipartisan vote is one in which a larger part of Republicans and a dominant part of Democrats vote a similar way. The term suggests that the choice is unimportant to the benefits of the current issue or the political convictions of individual individuals, yet rather is managed by party strategy.