It’s the free market, baby! Politicians, for sale to the highest bidder!

I don’t think amount of political favor is dictated solely by monetary contribution size. Though, there is a very very strong correlation.

I also don’t think a case could be made the unions have NOT been losing power over the last several decades.

Interesting snippet from Wikipedia

Although most industrialized countries have seen a drop in unionization rates, the drop in union density (the unionized proportion of the working population) has been more significant in the United States than elsewhere.