DES MOINES — Donald J. Trump has dominated much of this political season, excited an often-ignored part of the electorate, filled huge arenas with followers and upended the rules of how modern campaigns are run.

But now he faces an urgent question: Can he actually win crucial early contest states?

Translating a personality-driven campaign to the voting booth is no easy feat, especially for a candidate who has never run before.

But here in the state with the first nominating contest, about six weeks away, Mr. Trump has fallen behind in the nuts and bolts of organizing. A loss in Iowa for Mr. Trump, where he has devoted the most resources of his campaign, could imperil his leads in the next two nominating states, New Hampshire and South Carolina, where his get-out-the-vote organizations are even less robust.

A successful ground game is crucial in Iowa because of the state’s complicated method of caucus voting, but the Trump campaign has lagged in reaching some of its own benchmarks.