Secret Trump support may exist, so Clinton can't take lead for granted: Ex-Obama advisor

Round 3: Clinton vs. Trump

Donald Trump may have a point with claims that polls don't reflect the full extent of his support, said Mark Hannah, a former campaign advisor to President Barack Obama and a Hillary Clinton supporter.

"People respond to pollsters with what they think the pollster wants to hear sometimes," he told CNBC on Wednesday.

In the same "Squawk on the Street" interview, Republican strategist Charlie Black disagreed with Hannah, saying the nation's pollsters are "very astute," and there may be 1 to 2 percent of voters "secretly for Trump."

But Hannah countered: "I'm concerned that Trump might have this latent base of resentful support."

Clinton "needs to go for the knockout punch" in the third and final presidential debate on Wednesday night in Las Vegas, he added, saying it would be a "mistake" for Clinton to continue "waiting in the wings," as she's done in past debates.

The Democratic nominee should go after Trump for bragging about groping women on the leaked 2005 "Access Hollywood" video and allegations from women who said he treated them in a similar fashion, Hannah said.

"There's near universal assent that the behavior of Donald Trump in the past is discrediting to the office," he said, suggesting Clinton make an appeal for gender unity, like Obama did with a speech on racial equality in his first presidential campaign in 2008.

Black — who's has advised every GOP nominee from Ronald Reagan to Mitt Romney — said the key for Trump at Wednesday night's debate comes down to the issues. "[Clinton] represents the status quo. He represents change. And he needs to explain how he's going to make things better with economic policy, trade [and] immigration."

Trump is behind and he knows it, said Black, who was more recently an advisor to Ohio Gov. John Kasich's failed bid for the Republican nomination. "[Trump] was effective in August and early September in talking about issues and sticking to his teleprompter. He gained in the race. He needs to do that again. He still has time."

Black said he understands why some Republican leaders, including Kasich, may not support Trump, but he plans to vote for his party's nominee in the November election that's less than three weeks away.