Firebrand Sen. Ted Cruz is surging in the Republican primary race and effectively replacing Ben Carson as Donald Trump’s biggest threat as several polls show him claiming the front-runner mantle in Iowa.

The field shake-up comes ahead of the last GOP primary debate of the year, set for Tuesday, and appears to be stoking tensions between the Texas senator and billionaire businessman.

A Fox News Poll released Sunday showed Cruz edging Trump in Iowa, 28-26 percent among likely caucus-goers. The poll was taken Dec. 7-10, and had a margin of error of 4.5 percentage points.

Cruz registered an even larger lead in a Des Moines Register/Bloomberg Politics poll, opening up a 10-point advantage over Trump in the same state.  This would mark a 21-point surge since the last poll by the same outfit.

Cruz’ momentum is not isolated to Iowa, either. A Wall Street Journal/NBC News poll showed Cruz rising to 22 percent – up from 10 percent in October – and pulling just behind Trump’s 27 percent nationally.

The surveys mark a big shift from just a few weeks ago, when Carson, the soft-spoken retired neurosurgeon, was emerging as Trump’s biggest threat. But Carson faced questions about his experience and policy knowledge, and senators like Cruz and Florida’s Marco Rubio have since climbed.

Cruz’ ascent in the polls, though, has been far more dramatic than Rubio’s.

In the Fox News poll, Rubio clocked in at third with 13 percent, while Carson slipped to fourth with 10 percent.

When the results were narrowed to just those GOP caucus-goers who say they will “definitely” attend, Cruz’s advantage over Trump increases: 32 percent vs. 25 percent. Trump maintained a 3-point edge over Cruz among those who say they will “probably” attend.

“This is Trump’s challenge in Iowa,” said Democratic pollster Chris Anderson. “Those who are definitely going to caucus are less likely to support him, so he needs to motivate people to attend who may not usually do so.” Anderson conducts the Fox News Poll along with Republican pollster Daron Shaw.

Cruz gets his highest support -- 48 percent -- from those who are part of the Tea Party movement. That’s double Trump’s 24 percent.

Meanwhile, Cruz has more room than Trump to grow his support in Iowa.  Three in 10 caucus-goers say they could “never” support Trump (30 percent), while just 6 percent say the same about Cruz.

Cruz’ rise has fueled a back-and-forth with Trump, though the two have until now largely avoided scrapping. Cruz continues to hold back, but Trump on “Fox News Sunday” said he doesn’t think Cruz is qualified to be president.

“I don't think he has the right temperament. I don't think he's got the right judgment,” Trump said. “When you look at the way he's dealt with the Senate, where he goes in there like … you know, frankly, like a bit of a maniac. You never get things done that way.”

Cruz and Trump will be among the nine candidates on the main debate stage in Las Vegas Tuesday night. The debate will be hosted by CNN.

Fox News’ Dana Blanton contributed to this report.