AURORA, Colo.—The 2016 White House race has already shattered more than its share of precedents. Luanne Mitchell plans to break another when the 53-year-old lifelong Republican votes for Hillary Clinton this fall because she can’t bring herself to support Donald Trump.

About 10 miles away, Jim Benson is eager to celebrate his own political watershed when the 74-year-old Democrat casts his first-ever ballot for a Republican presidential candidate. “As soon as I heard Trump was running, I said, ‘That’s the guy I want,’ ” he...