Traditionally, the people most likely to show up on a cold, Iowa night for the first-in-the-nation nominating contests have previously attended caucuses. But if Democrat Bernie Sanders and Republican Donald Trump can persuade newcomers to turn out, they could gain substantially.

Each candidate is doing well among people who say they would be caucusing for the first time, while their chief rivals lead among people who say they have attended prior caucuses. The challenge for Messrs. Sanders and Trump: Teaching newcomers the unfamiliar mechanics of the caucuses requires a campaign to invest in training and outreach.