Party
A lot of my overseas friends used to find American politics baffling because, for most of the 20th century, US parties were about alliances, not ideas. In most places, a party has a core idea (Labor, Catholicism, Communism); in the US, it didn't. The Republicans were more conservative, but they had plenty of liberals. The Democrats had buckets of conservatives.
This wasn't a matter of degree; there were Republican liberals who were more Progressive than most Democrats, and the Democrats had a big block of the conservative reactionaries who were far to the right of most Republicans.
This has been changing, gradually, since 1964. Or maybe it started in 1948, when the segregationists stormed out of the Democratic convention to back Strom Thurmond. But it's been gradual.
That change is essentially finished now; the most progressive Republicans are now to the right of the most conservative Democrat. There are one or two outliers -- mostly old-line New England Republicans -- but they're obviously anomalous (and may be ready to switch parties).
It might be a good change, but it is a change. And it's going to make the US a more bitterly partisan place for the next few years than it's been in living memory.