Donald Trump's policy proposals would actually reduce economic growth, Glenn Hubbard, an advisor to rival Jeb Bush, said Wednesday, ahead of the third Republican presidential debate.
"Some of the candidates in this race are presenting economic flimflam to the American people," he said on "Squawk Box." "I would put Donald Trump definitely in that category."
Read MoreCarson momentum is top threat to Trump: Ex-GOP advisor
Hubbard, also a former chairman of the Council of Economic Advisers under President George W. Bush, described Trump's ideas on immigration, trade, taxes, and the budget as "unrealistic" and "simply dangerous."
The debate Wednesday night, sponsored by CNBC, should be a chance for Jeb Bush and other candidates, such as Sen. Marco Rubio of Florida and Ohio Gov. John Kasich to expose that, he said, adding Bush needs to demonstrate why his tax plan and other ideas would jump-start the economy.
"We need to have a serious debate over whether we're going to get stuck at 2 percent or grow much faster," said Hubbard, dean of the Columbia Business School. "We will not do that with the kind of policies Donald Trump is suggesting."
Before he even announced his candidacy, Bush was thought of as a shoo-in for the GOP nomination. But he's been lagging in the polls, with just a fraction of the support for Trump, who enjoyed front-runner status for more than 100 days.
A new national New York Times/CBS News poll Tuesday showed retired neurosurgeon Ben Carson edging out Trump for the lead. Carson also has been surging in the early presidential contest state of Iowa, leading Trump in most local polls there.
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Greg Mankiw, who followed Hubbard as chairman of the Council of Economic Advisers in the second Bush administration, told "Squawk Box" in a separate interview that Carson and Trump are not strong candidates.
"I think we've been too much focused on entertainment, the performance act of politics, and not enough on the policy," said Mankiw, contending Trump likes to talk about himself rather than substantive ideas. "I find it very entertaining too. But I don't think we should choose a president on the basis of who's a good entertainer."
Mankiw, who has not committed to any one candidate yet, said he likes Bush and Rubio, calling their ideas on the economy solid. But he's concerned the anger among Republican voters that's driving the outsider strength of Trump and Carson could be self-defeating.
"If the Republican Party nominates Trump or even Carson, who I have a lot of respect for as an individual ... we're going to lose to [Democratic front-runner] Hillary Clinton," said Mankiw, economics professor at Harvard University.
It's a long road to the GOP convention in mid-July. But Mankiw sees the primary race coming down to Rubio and Carson, with Rubio winning the nomination.
Voters are going to realize Carson is "kind of thin on knowledge of the issues" and Rubio is the best choice to sell the Republican message to the American people in the general election, he said.
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 5 p.m. ET.