Mr. Sanders and his campaign say their main logistical challenge now is building an infrastructure that matches the momentum seen in polls and rallies.

“We had assumed there would be a more gradual ramp-up,” Mr. Weaver said.

In Iowa, the campaign is hiring professionals to coordinate volunteers and their canvassing activities, but also to better build what Mr. Weaver called a “geographically diverse” base of support in the state. Already attracting young voters, the campaign is confident that older voters will also gravitate toward Mr. Sanders as they learn more about the 73-year-old’s positions on access to cheaper prescription drugs and on Social Security.

While Mrs. Clinton has an impressive Iowa organization, Mr. Weaver said that had not meant all the talent was locked up. With so many veterans of past Democratic campaigns available, there is a “whole cadre of people who are not in the Clinton world,” he said.

And the campaign has inherited some key operatives who worked for President Obama. Scott Goodstein, the founder of Revolution Messaging, which is handling the digital operation for the Sanders campaign, ran social media efforts for Mr. Obama’s 2008 campaign. Zack Exley, who joined the campaign in August, also did work for Mr. Obama’s 2008 campaign. And the president’s former site architect and videographer are also on board.

That is not to say the campaign’s field operatives sell themselves short. Mention the lack of a figure like David Plouffe, the organizing force behind Mr. Obama’s campaigns, and Pete D’Alessandro, the campaign’s Iowa director, says, “That’s me!”

Mr. D’Alessandro, the campaign’s second hire in the state and a former organizer for former Gov. Chet Culver, said the major crowds out West had energized volunteers in Iowa, who watched feeds of the events at campaign pizza parties.

He added that larger events in nearby states also expanded the volunteer base, and that the campaign had asked supporters in Madison, Wis., where Mr. Sanders drew a crowd of 10,000, to come knock on doors in Dubuque.