JACKSON, Miss. — The question seemed to tug at Senator Thad Cochran throughout his bumpy re-election campaign in Mississippi: Why, after a long, distinguished career — 42 years in Congress, including six terms in the Senate — was he seeking a seventh term? By all accounts, Mr. Cochran had needed to be talked into running. He told one national publication that he had planned to retire, and another that the first thing he wanted to do after Election Day was take a nap. And while the snowy-haired, 76-year-old Republican tried to hush whispers that he was not quite as sharp...