WAUKEE, Iowa — Democratic presidential hopeful Martin O’Malley, whose own poll numbers haven’t really budged since entering the race, says his rival Bernie Sanders is on the rise partly because voters see him as a “protest candidate.”

“I’m not running for protest candidate, I’m running for president of the United States,” the former Maryland governor said in an interview Thursday night following a campaign stop here that is part of a three-day swing through Iowa to highlight his proposals on climate change.

Sanders, a self-described democratic socialist who rails against the political influence of the “billionaire class,” has seen his crowds swell in recent weeks, and he is gaining ground on Democratic front-runner Hillary Rodham Clinton in polls in both Iowa and New Hampshire.

Sanders, an independent senator from Vermont, has said repeatedly that he is running to win the Democratic nomination and the presidency.

On Wednesday, he drew about 10,000 people to a rally in Madison, Wis., the largest of the 2016 cycle by any presidential candidate. In a poll of likely Iowa caucusgoers released Thursday, Clinton was leading Sanders, 52 percent to 33 percent. O’Malley, who aides say is using the summer to become better known, was at 3 percent in the Quinnipiac poll.

Asked what he makes of Sanders’s appeal, O’Malley said: “I think it shows the widespread desire for an alternative to this year’s inevitable front-runner.”

“It doesn’t terribly surprise me,” O’Malley said. “People feel like big money has subsumed, taken over, their politics, and they’re frustrated by it. … People feel like their voices don’t matter. People feel like they’re not being heard, and right now, they want to protest about that.”

Asked if he sees Sanders as a protest candidate, O’Malley said: “I think there’s an element of it … yeah.”

O’Malley said the popularity among Republicans of Donald Trump, the businessman turned presidential candidate, reflects a similar phenomenon.

On Thursday, Sanders’s campaign said he had raised $15 million during the last quarter. That’s about one-third of the amount Clinton’s campaign said she raised for her committee during the same period, but Sanders’s take is expected to far outpace what O’Malley and other Democrats in the field have raised.