DES MOINES — This fall the words of a Hillary Clinton backer were posted like a locker-room taunt on the wall at Bernie Sanders’s Iowa headquarters: “There is no Sanders organization.”

The quote, from Jerry Crawford, a leader of Iowa’s Democratic establishment, was meant to enrage and goad the Sanders staff, and it did the job.

The campaign has quietly assembled an extensive ground game here, with 100 paid staff members and with trained volunteer leaders for each of the state’s 1,681 caucus precincts.

The field team is meant to be the engine for a Sanders upset in the caucuses on Feb. 1 — the vehicle to turn out the tens of thousands of grass-roots supporters who show up for Mr. Sanders’s rallies, even if they no longer earn him headlines.