Republican presidential candidate Ben Carson meets with employees at Nationwide Insurance in Des Moines on Monday. (Scott Morgan/Reuters)

DES MOINES — Struggling to maintain momentum ahead of the Iowa Republican caucus, presidential candidate Ben Carson accused GOP front-runner Donald Trump on Wednesday of spreading "rumors" about his past that contributed to "unprecedented attacks" on his character.

Carson, who once led polls here in Iowa, predicted that Trump and Sen. Ted Cruz (Tex.), another GOP front-runner, will "probably" perform worse than anticipated because news organizations are not polling his universe of supporters accurately.

"A lot of the people who are supporting us are people who have not traditionally been in the political arena. And those are people who are not polled. You only poll likely voters," the retired neurosurgeon told reporters at a Bloomberg Politics Breakfast Briefing in Des Moines.

Facing a barrage of questions about Trump, Carson eagerly engaged, first by dismissing the businessman's plans to skip a presidential debate on Thursday night.

Here are the most surprising moments from Republican presidential candidate Donald Trump's rambling speech in Fort Dodge, Iowa on Nov. 12. The candidate spoke for more than an hour to an audience at Iowa Central Community College. (Sarah Parnass/The Washington Post)

"I wouldn’t be surprised if he did show up," Carson said. "But the key thing it gives us an opportunity to highlight is that this debate is not about personalities. It’s not about Donald Trump. It’s not about me or Ted Cruz or Mike Huckabee. It’s about the American people, it’s about what’s happening to our nation. And I personally believe that our nation is in great trouble."

[Trump says he won't participate in GOP debate on Fox News]

He grew especially upset Wednesday when discussing Trump's decision last fall to seize on news reports about Carson's past, including accounts about violent episodes from his childhood, about claims that he was offered a "full scholarship" to the U.S. Military Academy and about his time at Yale University.

Trump called the stories "a bunch of crap" and accused news outlets of touting details about the claims but later failing to promote clarifications and corrections to those stories as prominently.

But when reporters reminded Carson that Trump had helped fuel talk of the stories, Carson agreed, saying, "It says that he acts like a politician. Politicians do things that are politically expedient."

Asked whether he thought Trump had been dishonest, Carson said: "He was very dishonest."

Carson generally avoided direct policy contrasts with Trump or Cruz on Wednesday, saying that he preferred to focus on his own ideas.

"I haven’t heard a lot of policy from Mr. Trump. I hope that we will hear more policy. I hope that he will begin to talk about these things in more detail," the retired neurosurgeon said at one point. "I hope that he will talk more about his tax plan."

Despite his anger toward Trump, Carson hedged when asked whom he would support if he had to choose between the businessman and Cruz.

"One of the things that I've learned in this process is that there are some questions you don't answer. That's one of them," he said.

Close observers of Iowa's Republican caucus predict that at least 50 percent of attendees will be evangelical Christians, who form the majority of Carson and Cruz's supporters. But Trump has made inroads in recent days by scoring the support of Liberty University President Jerry Falwell Jr., former vice presidential nominee Sarah Palin and others who hold sway over the state's socially conservative voters.

When compared with Cruz and Trump, Carson said, "maybe I’m not" the best choice for evangelical voters. "That’ll be a decision they’ll have to make. It is persuasive to them because I’m getting out there in front of them," he said.

But he implored reporters not to draw early conclusions about the results.

On Monday night, he said, he "obviously would like to finish in the top three." If he doesn't, Carson said, he plans to reevaluate his campaign.

"There’s always a path, but you always have to look at the trends, you have to look at what is happening, what are people saying. And that’s what I will do," he said.