If pundits and observers are skeptical that Donald Trump can win a primary—much less the primary—it’s not because they don’t buy his popularity. That much is evident. He still leads national polling, he’s still ahead in early states like New Hampshire, and his supporters are well in his corner: 63 percent of Trump backers say their minds are “made up” about their candidate of choice.

They’re skeptical because it’s not enough to be popular. You also need the organizational strength to bring your backers to the polls. You need to convert your polling lead into tangible performance, and for a candidate like Trump—who pulls from groups that don’t often vote in primaries—that takes extra time, extra effort, and extra money. And despite his (alleged) wealth, Trump has been unwilling to spend the money it takes to actualize his popularity and make it more than just a number in a polling average. At most, he’s prepared to run ads. But ads don’t move bodies. Staffers, volunteers, and field operations do. To that point, in the 2012 general election—a different beast, but still instructive—the effect of ads disappeared after a short period. The effects of a good ground game, however, didn’t. “Other things equal,” writes political scientist John Sides, “Obama’s vote share was about three-tenths of a point higher in counties where Obama had one field office and six-tenths of a point higher in counties where Obama had two or more field offices.”