FORT DODGE, Iowa — After filling arenas on the coasts, reaching out to voters through social media and facing off with Hillary Clinton in nationally televised debates, Senator Bernie Sanders of Vermont began a critical leg of his surging campaign for the Democratic presidential nomination the old-fashioned way, traveling on Tuesday across frozen and whitewashed Iowa plains in a campaign bus.

“We got a new bus out there,” Mr. Sanders said, referring to the coach with the Obama-echoing slogan “A Future to Believe In” written on the side. Saying he would take the bus to meetings with voters all over the state in the two weeks left before the caucus, Mr. Sanders added: “It is very different than other buses that I have ridden on, let me tell you that. You can walk around — comfortable seats.”

A luxury bus driving him to Carroll and Underwood and Sioux City is not the only thing different for Mr. Sanders’s campaign. After closing the gap with Mrs. Clinton in Iowa polls and taking a lead in New Hampshire, the anti-establishment Mr. Sanders, a self-proclaimed democratic socialist who for months refused to bring a pollster onto his campaign staff, spoke about his positive numbers with the gusto of Donald J. Trump.