Carson momentum is top threat to Trump: Ex-GOP advisor

Donald Trump faces the risk that Ben Carson will corner the outsider vote if he wins the Iowa caucus in the race for the Republican presidential nomination, former GOP campaign adviser Kevin Hassett said Wednesday.

Carson recently overtook Trump as the front-runner in a number of polls, just in time for Wednesday night's third Republican debate, sponsored by CNBC. CNBC's full coverage begins at 5 p.m. EDT.

Read More Poll Watch: Ben Carson Edges Ahead Nationally in Times/CBS News Poll

"I think a lot of people are now looking at Iowa, and thinking that Carson's going to win Iowa. Then all of a sudden the nonestablishment people coalesce around Ben," Hassett told CNBC's "Squawk Box." "I think that's the Number 1 threat — near-term threat — to Trump."

For that reason, Trump will likely try to distinguish himself from Carson in Wednesday's debate, Hassett said.

Wednesday night's main forum essentially will highlight two sets of candidates, said Joe Lockhart, former press secretary to President Bill Clinton. One is focused on political outsiders like Carson, Trump and former Hewlett-Packard CEO Carly Fiorina, and the other is between candidates with governing and legislative experience.

In the latter, candidates must still differentiate themselves from Trump, Lockhart told "Squawk Box." "There will be two debates, but Trump will be in the middle of both of them."

Read MoreRepublican presidential debate

Trump has the most to lose in the debate because he has dominated the campaign narrative, Lockhart said. However, the focus has not been on substantive issues like taxes and health care, but the trading of insults between Trump and the other candidates, he added.

"I expect other Republican candidates will really be trying to put him on the spot and demonstrate that when it comes to policy, he tends to be making it up as he goes along."

Donald Trump
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Donald Trump

Tough economic policy questions will provide an opportunity for someone who is adept in that field to shine at the CNBC debate, which will emphasize economic and financial issues, Hassett said. He noted that Sen. Marco Rubio and former Florida Gov. Jeb Bush have attempted to run substantive, policy-based campaigns.

Lanhee Chen, director of Domestic Policy Studies at the conservative Hoover Institution, said he expected Rubio to be able to present a forward-looking vision on the economy and foreign policy.

The focus on economic issues at the debate will "expose the true contenders from the wannabes," he told "Squawk Box."

As for Bush, the conventional wisdom is that he needs a break-out performance, said Jared Bernstein, former economic policy adviser to Vice President Joseph Biden.

"He's been really pretty ineffective in these debates so far and kind of riding on an assumed front-runner status," Bernstein told "Squawk Box."

Read More GOP candidates on retirement and student debt

Bush has only 5 percent support among likely Republican caucus participants in Iowa, according to the latest Quinnipiac Poll, released Thursday. The poll surveyed 574 Republicans by phone and has a margin of error of plus or minus 4.1 percentage points.

The prospect of Trump or Carson making it all the way to the Republican National Convention can't be ruled out because the 2016 race is being run with "a very different set of rules," Bernstein said. So far, it has been an outsider's game, he noted.

In that regard, Bernstein added, Rubio is interesting because he fits somewhere between the political insider and D.C. outsider models.

Watch CNBC's "Your Money, Your Vote: The Republican Presidential Debate" tonight. The debate will feature two sets of candidates discussing critical issues facing America today, including job growth, taxes and the health of our economy. Coverage begins at 5pm E.T.

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