Lt. Gov. John Fetterman of Pennsylvania and Mehmet Oz, the celebrity television doctor, collided Tuesday in one of the most closely watched debates of the midterm campaign. The Pennsylvania Senate matchup was both highly personal and unusual, as viewers watched Mr. Fetterman, a Democrat, rely on closed captioning to accommodate for the lingering effects of a stroke he had in May. Mr. Fetterman set out to show Pennsylvania voters that he is ready to serve — and to take on a Republican opponent who has recently gained ground in a race that could decide control of the Senate. He repeatedly...