Inside the Rubio headquarters here, there are ample signs of a campaign that is sprinting to the finish.

The detailed demographic breakdown of likely New Hampshire voters tacked to the wall, the tallies of thousands of phone calls made and doors knocked on, and the rankings of volunteers who have exceeded targets all point to a largely unnoticed effort to build a network of support.

Ryan Fattman, 31, a state senator from central Massachusetts, said that since July he had recruited about 350 volunteers from across his state to help campaign for Mr. Rubio in New Hampshire, part of a broader effort to mobilize state and local politicians from across New England.

The result, he said, has been a volunteer-based network that has been working quietly for months. “I think their ground game has been kind of a hidden secret,” he said. “Part of the campaign’s brilliance is not advertising everything it’s doing.”