If the Democrats had seen a sizable percentage of their voting base swinging left anytime in the last 30 years, then they wouldn’t have been so quick to try the failed strategy of courting centrist Republicans.

I'm a third-party voter, and Democratic party hater, but I'm not sure this is true. The Democrats have repeatedly shown that they would rather let the Republicans take control, than let Leftists have a shot.

I agree with the rest of what you said though