1. A politician who represents corporate interests and wealthy individuals rather than the interests of middle and working class Americans and chooses to run as a Democrat rather than Republican either because his state or district will not elect a Republican or he has convinced the wealthy he would do a better job protecting their money than the GOP alternative. 2. A Democrat who puts the interests of corporations and wealthy individuals ahead of the progressive ideals of the Democratic Party and ahead of even the success of the party as a whole. 3. A Democrat who calls himself a moderate when in fact he is an extremist in protecting moneyed interest and is to the right of the overwhelming majority of the American people and even his own constituents.
The difference between a Republican and a Blue Dog Democrat is the same as that between an Almond Joy and a Mounds. Republicans have nuts (religious & gun-toting racists), Blue Dogs don't.