While Donald Trump leads the pack in recent GOP polls, the strength of Republican voters' allegiance to him is less clear. According to a new national Monmouth University poll, Trump remains firmly on top of the Republican candidates, but he loses when he goes head-to-head with Ben Carson.

The new poll shows Carson gained traction Thursday and stepped into second place with 18 percent of the votes -- up from 5 percent in the last Monmouth poll released before the Republican debate.  

His campaign's viability could pose a threat to Trump, should the party's field become the two of them. When voters were asked to choose between them, the surgeon won 55 percent of the votes to Trump's 36 percent.

But the poll still left Trump with some new bragging rights: He scored 30 percent of the votes in the Republican field. With the exception of Carson, the real estate mogul beats every other Republican candidate in head-to-head matchups. 

“None of the establishment candidates is having any success in getting an anti-Trump vote to coalesce around them. In fact, any attempt to take on Trump directly only seems to make him stronger,” said Patrick Murray, director of the independent Monmouth University Polling Institute in West Long Branch, New Jersey.

Monmouth's findings are on trend with other recent polls, which have seen Carson's support surpassing former Gov. Jeb Bush, who dropped to third place with 8 percent of the vote. Bush was down from 12 percent in August, although the dip is within the poll's margin of error.

Wisconsin Gov. Scott Walker's ratings also dropped. He maintained third place in the August poll, but was down 8 points to 3 percent.

Florida Sen. Marco Rubio came in fourth on the list at 5 percent, and former Arkansas Gov. Mike Huckabee and Carly Fiorina tied at 4 percent.

The university interviewed 366 Republican voters by telephone from Aug. 31 to Sept. 2 for the poll, which has a margin of error of plus or minus 5.1 percentage points.