DES MOINES, Iowa—At a small dinner party recently, several supporters of Hillary Clinton kept returning to a two-word phrase that represented their biggest fear—enthusiasm gap.

That same day, her rival Donald Trump spoke to a rally of 8,000 in Florida, according to local media reports. People waited in line for hours in the 95-degree heat for a glimpse of the Republican nominee in what by now has become a familiar pattern. On the trail, Mr. Trump invariably is the bigger draw.

“I’ve never seen anything like this enthusiasm,” said GOP vice-presidential nominee Mike Pence. “Everywhere I go, even I’m getting big crowds, and I’m not the main event.”

Whether enthusiasm levels make a difference in the election is one of the key tests of the unpredictable 2016 presidential race. A campaign’s organizational muscle can be crucial to converting the energy at a rally into something tangible—volunteers, donors, votes—and that is what the Trump campaign is seeking to do.

At the same time, even lukewarm Clinton voters may feel ample motivation to cast their ballots if their disdain for the Republican nominee runs deep enough, her allies say. And large crowds don’t necessarily translate into votes. During the Democratic primaries, Vermont Sen. Bernie Sanders attracted far bigger audiences than Mrs. Clinton but lost handily.