Racism is any time you expect a person to believe or behave a certain way because of their race. In this case, comments such as 'Uncle Tom' do exactly this. If you are black, you are expected to hold to liberal beliefs and are expected to vote Democratic. This isn't any different than when blacks (slaves) were expected to believe they were inferior and to behave in the way their owner desired. Also, the phrase 'Uncle Tom' is never applied to other than blacks, another indication this is a racist term.
This guy has been getting away with this crap for years. He deliberately says things to inflame his listeners and to generate phone calls. This town is a big morass of ultra-left wingers. They will cut this guy slack, since he spews the same liberal/regressive hypocritical nonsense they do.