Final answer:
The Chicago Cubs hold the longest ongoing championship drought in major league U.S. men's sports, with a drought of 73 seasons since their last championship in 1908.
Step-by-step explanation:
The team that holds the longest ongoing championship drought in major league U.S. men's sports is the Chicago Cubs. They started out in Chicago in 1876 and have not won a championship since 1908, resulting in a drought of 73 seasons.