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With just two weeks until the Iowa caucuses, Senator Bernie Sanders of Vermont is hitting the road and stepping on the gas.

On Tuesday morning, with the sun hardly up and the cold hardly bearable, Mr. Sanders will leave Des Moines by bus for a day of events in Fort Dodge, Carroll, Pottawattamie and Sioux City. Mr. Sanders has closed the gap with Hillary Clinton in Iowa, a state that helped propel Barack Obama in 2008, and which is critical to Mr. Sanders’s chances in 2016.

His campaign is confident that a surge in enthusiasm among young voters, his improved debate performances, and a well-built get-out-the-vote organization has given him momentum headed into the homestretch.

His travels on Tuesday are an effort to solidify those gains on the ground.

Mrs. Clinton, who came in third in the state in 2008, a crushing result that was the fatal flaw in the base of her campaign, is trying to put the brakes on Mr. Sanders. In the debate on Sunday, she echoed the attacks her campaign has made on Mr. Sanders’s integrity and electability by questioning his positions on guns, Wall Street and synchronicity with Mr. Obama.

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