Sanders has mounted a formidable small-donor fundraising machine. His campaign last week announced it raised $25 million in January from nearly 650,000 donors — his best fundraising month of the campaign.

Picking up on a line of attack he used in Friday’s debate in New Hampshire, Sanders slammed Buttigieg on CNN’s “State of the Union.”

“I will also point out that I am running against a candidate, Pete Buttigieg, among others, who has raised contributions from more than 40 billionaires, including the CEOs of some of the largest drug companies in America,” Sanders told host Jake Tapper.

The former mayor of South Bend, Ind., said he’s “building a campaign that’s not defined by who we reject.” | Win McNamee/Getty Images

The former mayor of South Bend, Ind., defended himself Sunday, telling Wallace that he’s “building a campaign that’s not defined by who we reject.”

“We’ve got 2 million contributions in this campaign; I think the average is under 40 bucks,” Buttigieg said Sunday. “We’re building the movement that is going to defeat Donald Trump. I want everybody to help out. I want everybody who shares that vision to be at our side.”

On CBS’ “Face the Nation,” Buttigieg made it clear he wasn’t for sale.

“I've never hesitated to stand up to industry. We sued the pharmaceutical industry when opioid-makers ravaged our community, and I'm campaigning right now for higher taxes for the wealthy and for corporations to finally have to pay their fair share,” he said.

Sanders and Buttigieg are running neck and neck in the still-too-close-to-call Iowa caucuses, which were mired last week by technological and organizing failures, and polls released late last week show a tight race between the two in New Hampshire.