Maine's U.S. Senate race is a battle between not two, but three candidates vying to replace retiring Senator Olympia Snowe: Republican Charlie Summers, Democrat Cynthia Dill, and former Governor Angus King, a left-leaning independent. Despite outside groups like the U.S. Chamber of Commerce spending millions attacking King, he retains a sizable lead -- but the complex, multi-million dollar web of outside spending has made Maine's U.S. Senate race one of the most interesting in the country.