Complex sentence: While each of the three leading candidates (Gary Johnson, Austin Petersen and John McAfee) had been invited, none attended the debate, but other candidates Marc Allan Feldman, Darryl W. Perry and Kevin McCormick.
Split into simpler sentences: While each of the three leading candidates (Gary Johnson, John McAfee, and Austin Petersen) had been invited, none attended the debate. Attending instead were candidates Marc Allan Feldman, Darryl W. Perry, and Kevin McCormick.