LAS VEGAS—Robbee Brown can’t wait to attend a caucus for Donald Trump, if only someone would tell her how.

Ms. Brown, a lifelong Republican, wore a button with Mr. Trump’s photograph to his Monday night rally here. But the 61-year-old blackjack dealer said she’s never been to a caucus. “I don’t know when it is,” she said. “I have not heard from one person telling me where to go.”

Educating...