“How are we going to pay for it? Are we going to pay for it in the form of still further taxes or are we going to pay for it in the form of broken promises?” Mr. Buttigieg told supporters Monday at Plymouth State University in Plymouth. “We have to choose a responsible approach that can actually get big things done and answer the question of how we’re going to get from point A to point B.”

The two candidates have not just implicitly drawn contrasts; they have also attacked each other directly, especially in the last few days as the Iowa results and recent polls established them as the two front-runners. Over the weekend, Mr. Sanders laced into Mr. Buttigieg for raising money from billionaires, including “heads of large corporations, C.E.O.’s in the pharmaceutical industry, people from Wall Street.”

“If you think that people are going to get money from the C.E.O.’s of drug companies and are going to tackle the greed and corruption of the pharmaceutical industry, you’re mistaken,” Mr. Sanders told a capacity crowd in Hanover on Sunday. “If you think people are going to get money from financiers in the fossil fuel industry, and they’re going to be prepared to take them on and transform our energy system away from fossil fuel to energy efficiency and sustainable energy, you’ll be wrong. So, it does matter where money comes from.”

On Monday, he reprised his targeted jabs at Mr. Buttigieg, whom he called a “smart guy.”

“Pete seems to think that it doesn’t matter that, in his case, he raised lots and lots of money from at least 40 billionaires,” Mr. Sanders said at an afternoon event for supporters in Salem, at a coffee shop that featured a Sanders “presidential blend.”

On Saturday morning, before Mr. Sanders had begun his billionaire broadsides, Mr. Buttigieg was obliquely criticizing Mr. Sanders’s calls for a “revolution” as divisive. Though he never mentioned his rival by name, Mr. Buttigieg told a crowd of nearly 1,000 in Keene on Saturday morning that he disagreed with the notion that “if you’re not for a revolution” then “you don’t fit.”

On Monday morning in Plymouth, he stressed the point harder.

“This is a moment for bringing as many people as we can into the picture, but a picture where your only choices are between a revolution or the status quo is a picture where most of us don’t see ourselves,” Mr. Buttigieg said.