The big change is that nowadays the parties have realigned along ideological lines. Before the 60s, the Democratic party ruled the South. So the Democrats were a coalition of conservative segregationists, minorities, labor, and urban machines. Republicans were a coalition of various Northern interests, especially the middle class and industry, but they included both conservatives and classical liberals. Republicans were nonexistent south of the Mason-Dixon line.
So this meant that a conservative southern Democrat often had much more in common with a northern rock-ribbed Republican than they had with a northern Democrat. And so legislation was often “bipartisan” because both parties lacked national consensus. Party discipline was non-existent, and members of congress would vote according to the interests of their constituency back home.
Then along came civil rights, and white southerners abandoned the Democratic party, and shockingly joined the party of Abraham Lincoln and abolition, something that would have literally given heart attacks to their grandparents.
So now, instead of both parties being grab-bags of people with wildly different interests and ideologies, the Republicans became exclusively conservative and very southern, while with the loss of the southern conservatives the Democrats became exclusively liberal and very northern. And so we have the modern phenomenon of “red states” and “blue states”. In the past all those southern conservative “red states” nevertheless elected Democrats to congress, but no more.
And so the modern political parties began to mean something ideologically. It used to be that the parties were like sports teams, and you wanted your team to win because it was your team, but your team contained all kinds of people, some of whom you liked and other that you didn’t. And the opposing team was just the opposing team, and contained lots of people you liked and admired because they had a similar ideology to you, and even though there were other people you didn’t like, you couldn’t afford to get to nasty because you never knew when they’d turn around and support your issues.
Except now, with the realignment complete, you don’t get conservative southern Democrats supporting the conservative Republican president, because there aren’t any conservative southern Democrats anymore. You don’t get New England Liberal Republicans supporting the liberal Democratic president anymore, because there are no more liberal Republicans. And so we now have party discipline, and you support your party no matter what, because the other guys aren’t just the opposing team, they’re the enemy who want to destroy the country, and they’re all conveniently labeled for you with a D or an R.