The Book Explains... (spoiler alert)

why Spencer Tracy, the abler candidate, lost. While several here have noted that Skeffington hadn't connected with the younger voters, the issue is revealed as more complicated in the book. Basically, the old party machines that Skeffington represented had their support undercut by the New Deal. With Social Security, unemployment insurance, welfare and other programs, the poor were not looking so much to the ward politicians for help when things went bad.

I got a feeling the studios were trying to keep the politics as neutral as possible, hence the labor-union conflict was left out. Including it might've made the film overlong but the fact Skeffington and his opponents were both oppose to the losing Italian labor leader was illuminating.

McCluskey's TV spot was also treated differently. While it was every bit as fake in the book as in the movie, (rented dog, prepared dialogue and exploiting the cute kids), it was not the inept production disaster. McCluskey is more like Dan Quayle or Sarah Palin before they got media scrutiny.

There were other differences but these were worth knowing.