All of a sudden, there is a new Republican presidential frontrunner. For the first time, erstwhile leader Donald Trump has been overtaken by the fast-rising GOP star: Ben Carson.

In the latest CBS News/New York Times Poll released ahead of Wednesday's third Republican presidential primary debate, Carson is now leading nationally with 26 percent voters' support to Trump's 22 percent.

This developed as Christian leaders in Iowa stood up for Carson's faith after Trump appeared to question the Seventh-Day Adventist Church in his remarks last weekend.

Pastor Mike Demastus of the Fort Des Moines Church of Christ said Trump criticising someone else's faith is "laughable."

"This is a guy who can't even quote a Bible scripture to someone," he said in an article in USA Today.

Trump, who had been leading the field nationally and in key states since summer, on Tuesday acknowledged that Carson is gaining strength. "Ben Carson is now doing well," he said in an interview on MSNBC.

Expectedly, Trump vowed to attack Carson at Wednesday's primary debate in a bid to recover lost ground.

Trump used to refrain from attacking Carson in his campaign speeches. However, last weekend he abruptly changed his tone calling Carson "low energy" and questioning his immigration stance.

Trump now expects Carson to be attacked from all directions. "One thing I know about a frontrunner, you get analysed 15 different ways from China. A lot of things will come out," he said.

Trump still believes that in the end the Republican candidate in the November 2016 US presidential election will still be him even as he expressed "surprise" at some polls showing Carson ahead.

"I have tremendous crowds and tremendous love in the room and, you know, we seem to have hit a chord. But some of these polls coming out, I don't quite get it. I was No. 1 pretty much in Iowa from the beginning, and I would say we're doing very well there. So I'm a little bit surprised," he said. "The other polls, as you know, in other states are extraordinary."

The latest Iowa poll conducted by Monmouth University and released Monday showed Carson leading Trump by 14 points, his biggest lead yet.

The CBS News/New York Times survey showed the other GOP candidates trailing far behind Carson and Trump. Florida Sen. Marco Rubio was in third with 8 percent, followed by former Florida Gov. Jeb Bush and former HP chief Carly Fiorina with 7 percent.

The poll of 575 GOP primary voters was taken Oct. 21-25, and had a margin of error of 6 percentage points.

Speaking at a rally on Saturday, Trump said: "I love Iowa. And, look, I don't have to say it, I'm Presbyterian. ... Boy, that's down the middle of the road, folks, in all fairness. I mean, Seventh-day Adventist, I don't know about. I just don't know about."

Carson has called on Trump to apologise for his statement, but Trump refused to back down. "I would certainly give an apology if I said something bad about it, but I didn't. All I said was, 'I don't know about it,'" he told ABC's "This Week."

Now, even Christians who don't support Carson are coming to his defence. "He cast questions about Carson," Andrea VanBeek, who supports Jeb Bush, said. "In some ways, he's pretty smart in the way he says things without saying them, you know."

Some Christians do have questions about the Adventist faith, including their teachings that the Sabbath is observed on Saturday and what some see as their over-emphasis on the teachings of church founder Ellen White.