1 of 2
Joel Page, Associated Press
Independent candidate for the U.S. Senate, Angus King speaks with electrician James Sloan outside of Bath Iron Works, Monday, Oct. 1, 2012 in Bath, Maine.

PORTLAND, Maine — Tax returns show independent Maine U.S. Senate candidate Angus King earns far more money than the other two leading contenders in the race.

The self-made millionaire was challenged by one of his opponents and released seven years' worth of federal income tax returns on Monday. The returns show he and his wife earned on average nearly $570,000 a year and paid an average of 22.2 percent in federal taxes during the period.

Republican Charlie Summers and his wife averaged around $90,000 in annual income over eight years of federal tax returns his campaign provided.

Democrat Cynthia Dill and her husband averaged about $65,000 in income over 10 years' worth of tax returns provided late Monday.

Dill challenged the top candidates to release their tax returns in an effort to promote transparency.

Republican U.S. Sen. Olympia Snowe is retiring.