John Sides posts these useful graphs:
As John writes, “The party loyalty of Democrats has been increasing over time and has essentially hovered at 90% since 1992. (And Republicans are similarly loyal to the Republican nominee.)” Here’s the story from the 2000 election:
This shows the improvement in prediction given party ID and also demographics and political ideology.
The short story: voters are more predictable than they themselves realize.
P.S. John’s graphs are fine, but the y-axis shouldn’t go below 0 or above 100%.